The linguistic meaning of Bid’ah is anything new the previously did not exist. The Shari’ah terminology however means anything that has been introduced into the beliefs and actions of the Muslims that is in direct opposition to the Qur’an or the Sunnah, or anything that distorts the Sunnah or claims to be better than the Sunnah.
The Prophet ﷺ has cautioned us against us against any innovation [Bid'ah] that opposes the Qur’an or the Sunnah in the following two Ahaadeeth:
The Holy Prophet ﷺ said:
“Whoever innovates something in this matter of ours [i.e. the Islamic Creed] that is not a part of it, will have it rejected.” 
The Holy Prophet ﷺ also said:
“Every innovation is misguidance” 
These two Ahadeethth should be understood as every innovation introduced into the Islamic creed is misguidance. This is due to the fact that nobody can enlighten us more than the last Messenger ﷺ did. He was the last Prophet and the religion of all the Prophets was completed with him ﷺ. Allah (سبحانه وتعالى)says in the Qur’an:
“This Day have I perfected your Religion for you, completed My favor upon you, and have chosen for you Islam As your religion” 
The Prophet ﷺ taught us all we need to know about Allah (سبحانه وتعالى), and he also taught us our limits. Nobody can take away from what was revealed to him, nor can they add anything to it. Every creedal formulation unknown to the Prophet ﷺ and his Companions that was later introduced is in the fire. This is confirmed from hadeeth of Mu’aawiyah Bin Abu Sufyaan (رضي الله عنه) who narrated that the Messenger of Allah ﷺ said:
“Those people of the book who came before you split into seventy two sects, and soon this religion [Islam] will split into seventy-three sects. Seventy-two of them will be in the fire and one will be in Paradise, and that is the Jamaa’ah” 
In a hadith of similar import, the Holy Prophet ﷺ was asked who are the saved group? The Holy Prophet ﷺ responded:
“Those who are upon that which me and my companions are upon today” 
Those that departed from the Ahl us-Sunnah wal-Jamaa’ah only did so because they invented new beliefs. Creedal innovations must be avoided at all costs. One must always seek and then adhere to the known position of the Ahl us-Sunnah wal-Jamaa’ah.
There are also innovations of actions, however this is to be treated separately as some innovated actions have been encouraged if they are beneficial and do not contradict the Shari’ah at any point.
Imaam Shaafi’s (رحمه الله) understanding of the hadeeth “Every innovation is misguidance [Kulla Bid'ah dalaalah] is enough to demonstrate this point. He said:
“There are two types of Innovation [bid'ah]: the praiseworthy and the blameworthy. Whatever is accordance with the Sunnah is praiseworthy and whatever vies with it is blameworthy…The new matters are of two: Whatever differs with the book of Allah, the Sunnah, the Athaar, or the Ijmaa’a, then that is an innovation of misguidance [bid'at ul-Dalaalah] and whatever new matter from amongst that which is good and does not differ with anything from that [Qur'an, Sunnah, Athaar, Ijmaa'a] then it is a new matter that is not blameworthy” 
Ibn ‘Abd us-Salaam (رحمه الله), a Shaafi’ scholar wrote in his Qawaa’id:
“Innovation is of five types:
- Al-Waajibah: Such as the study of ‘Arabic syntax which helps to understand the speech of Allah and His Messenger because the preservation of the Shari’ah is obligatory [waajib] and nothing comes to us except by that being from the foremost rank of the waajib. Such is the much needed explanation of what is difficult to understand, the codification of the science of Fiqh [Usul ul-Fiqh] and working to distinguish between the authentic and rejected traditions.
- Al-Muharramah: Whatever opposes the Sunnah such as the false innovated beliefs introduced by the Qadariyah, Murji’ah, and the Mushaabihah [and any other astray sect that has innovated something into the creed]
- Al-Mandub: Every excellent goodness that was not established during the lifetime of the Holy Prophet [for example], the gathering for the Taraweeh prayers, the building of schools, spiritual retreats, speeches regarding the praiseworthy aspects of tassawuf… and anything else done purely seeking the Face of Allah.
- Al-Mubah: Such as the shaking of hands after the Fajr and ‘Asr prayers, having an abundance of assorted foods, drinks, clothes, and a wide house…
- However some of the mentioned Mubah things can also fall into the 5th category which is disliked [Makruh] or even not preferred [Khilaaf ul-Awlaa]” 
Imaam Abu Bakr al-Aajuri (رحمه الله), the great Shaafa’i scholar said in his Kitaab us-Shari’ah:
“Innovations [in the Creed] are misguidance, being that which opposes the Book and the Sunnah, and differs from the statements of the true believers.” 
Imaam ‘Abdur Rahmaan Ibn al-Jawzi [rahimhahullah] Tablees Iblees said in hisTablees Iblees:
“Bid’ah…refers to something which did not exist and was then invented. And, in most cases, innovations conflict with divine law by implying a need for human additions or deletions. Even an invented practice which did not contradict the Sharee’ah or imply any change was disliked by the majority of early scholars. They used to avoid any innovation, even though some types were allowable…It is thus, clear that that the early generation of Muslims cautiously avoided all innovations which even had the remotest connection to the religion for fear of changing the religion to the slightest degree. However, there were some new practices which did not contradict the Sharee’ah or change it; these practices were allowed” 
We can deduce from the given evidences that there are two types of bid’ah, praiseworthy and blameworthy. Let us now look at some praiseworthy innovations that have served great benefits for this Ummah.
One such innovation was the compilation of the Qur’an into the form of a book as done by Zaid ibn Thabit (رضي الله عنه), upon the suggestion of ‘Umar (رضي الله عنه), and command of Abu Bakr (رضي الله عنه). Even though Abu Bakr (رضي الله عنه) was hesitant out of fear because this was something the Prophet ﷺ did not do, ‘Umar (رضي الله عنه) encouraged Abu Bakr (رضي الله عنه) By Allah, it is good”. 
Another example of good innovation was the gathering together of the people to pray twenty units of Taraaweeh prayers behind one Imaam in the month ofRamadaan . This was done by ‘Umar (رضي الله عنه) after he had noticed that the people had their own separate group prayers throughout the Masjid. Rather than have a divided Jamaa’ah reciting the Qur’an all at the same time, he knew it would be best to unify them behind one Imaam and one recitation. It was even said, “This is an excellent innovation”. 
The Issue of Milaad un-Nabi [the Birth of the Prophet ﷺ
One innovation which is often attacked as being a bid’ah of deviation is theMilaad un-Nabi [the act of celebrating the birth of the Prophet ﷺ]. It cannot be considered as a bid’ah that opposes the Qur’an and the Sunnah as it has been proven [by scholars such as Imaam as-Suyuti] that it has roots in the Shari’ah as it is considered an act of loving the Holy Prophet ﷺ. Many people mistake this event as celebrating the Prophet’s ﷺ birthday and upon this folly they wrongfully equate the celebration with the western tradition of celebrating birthdays. However, what the believers are doing is celebrating the birth and the life of the Holy Prophet ﷺ and they are simply rejoicing in the fact that Allah has sent him as a mercy to the universe. In such a pure intention, one will indeed find merits.
Imaam Ibn Taymiyah (رحمه الله) attested to this fact when he said:
“…Likewise, what people have innovated by analogy with the Christians who celebrate the birth of Jesus (عليه السلام), or out of love for the Prophet (صلى الله عليه وسلم), and to honor him, may Allah reward them for this love and effort. Not for the innovation itself [but the love for the Prophet]…To celebrate and honor the birth of the Prophet (صلى الله عليه وسلم) taking it as a honored season, as some people are doing, is good and in it there is a great reward, because of their good intentions in respecting the Prophet (صلى الله عليه وسلم)”
It is the opinion of Imaam Ibn Taymiyah (رحمه الله) that there is great reward in celebrating the Holy Prophet’s ﷺ birth, as it is an act of honoring him. Imaam Ibn Taymiyah (رحمه الله) is not alone in this opinion. Imaam Ibn Rajab al-Hanbali(رحمه الله) in his Lata’if al-Ma’arif  strongly encouraged people to gather in the month of Rabi` al-Awwal to learn about the Holy Prophet’s ﷺbirth and life. Therefore, in the Shari’ah, the celebration of the Holy Prophet’s ﷺ birth falls under the category of praiseworthy Bid’ah, as is recommended [mandub].
However, a word of caution concerning a danger that poses as a threat to the unity of the Muslims of today. This danger is the misuse of what is initially a well intended beautiful celebration, downgraded to a judgmental criterion to mete out the “true believers” from the “hypocrites”, the “Sunni Muslims” from the “deviants” where somebody who does not partake in the celebration is wrote of as a deviant. This is wrong due to the fact the Prophet ﷺ, the Sahaabah, and the Tabi’een never practiced the Milaad in the form we find it today. The celebration of the Holy Prophet’s ﷺ birth must not be made obligatory, or anyway binding upon the Muslims.Let us take a lesson from history and Allah’s (سبحانه وتعالى) Speech in which He said:
“We sent after them, Our Messengers, and we sent Jesus, the son of Mary. We gave him the Injeel and We placed compassion and mercy in the hearts of those who followed him. They have innovated Monasticism. We did not make it obligatory upon them. They invented it to please Allah thereby, but they did not observe it with right observance. We gave those who believe from amongst them their reward, but many from them were rebellious sinners” 
The first and foremost exegete, Ibn Abbas (رضي الله عنه) has this to say in regards to this verse:
“The Christians built monasteries and cloisters to escape the persecution of Paul, the Jew. Allah did not make monasticism obligatory upon them. They did not innovate it but to seek the pleasure of Allah and had Allah enjoined it upon them they would not been able to give it’s due. Allah will reward the monks who did not contradict the religion of Jesus double for their faith and worship. Twenty Four of these monks resided in Yemen and when they heard of the Prophet they believed in him and joined his religion. However many of the monks were disbelievers, vying against the religion of Jesus” 
The quoted ayah from the Qur’an along with the explanation of Ibn Abbas (رضي الله عنه) shows us that Allah (سبحانه وتعالى) does indeed reward people for good innovation, however, when they start making it obligatory as something that has to be done in the religion then this becomes a punishable offense. Therefore anybody who celebrates the birth of the Holy Prophet ﷺ must do so with a pure intention of loving the Prophet ﷺ and should not make this binding upon other people.
 Saheeh al-Bukhaari, vol. 5, hadeeth no. 2697, Saheeh Muslim vol. 6, hadeeth no. 1718
 Sunan Abu Dawud, Kitaab us-Sunnah hadeeth no. 4607
 Surat ul-Maa’idah (5), ayah. 3
 Sunan Abu Dawud, Kitaab us Sunnah hadeeth no. 4597
 Tirmidhi hadeeth no. 2129
 You will tend to find that those that oppose this statement often fire back merely saying “the alleged statement of Imaam ash-Shaafi [Rahimahullah]’” without any shred of evidence to suggest that the statement of Imaam ash-Shaafi’ is fabricated, etc. They simply think they can refute the position of Imaam ash-Shaafi’ by peppering it with the description of being “alleged”. In-fact it is them that are doing the “alleging” – making allegations that scholars such as Imaam Ibn Hajar al-Asqalaani and the isnaad he quoted from contains a liar. Where are their proofs for such an implicit allegation?
 Fath ul-Baari, vol. 13, Kitaab us Sunnah, pp 370-372 Also mentioned by Imaam Ibn Rajab al-Hanbali is his Jami’ al-‘Ulum wal-Hikaam, commentary to hadeeth no. 28.
 Fath ul-Baari, vol. 13, Kitaab us Sunnah, pp 370-372
 The true believers referred to are the Imaams of the Muslims such as Imaam Maalik, Imaam Shaafi’, Sufyaan at-Thawri, Imaam al- Awzaa’i, Ibn al-Mubarak, Imaam Ahmad bin Hanbal, al-Qaasin bin Salaam and whoever followed their way” See Kitaab ash-Shari’ah, vol. 1, p. 236
 Tablees Iblees, translated by Bilal Philips, p.21-23, al-Hidaayah Publishing. Arabic Edition, see p. 37 onwards What can we deduce from this statement?
1. Innovations conflict with divine law in most cases, but not in all cases.
2. Innovations that did not conflict with divine law were disliked, although not forbidden.
3. They were disliked by the majority of the Salaf, indicating that some from amongst the Salaf actually opined innovations that did not conflict with divine law as good [bid’ah hasanaah].
4. It is solely the choice of an individual Muslim to avoid “bid’ah hasanaah” as this is a valid position and there is also merit in avoiding innovations for the sake of Allah, a Muslim should not feel bound to practice such and such due to the customs of his people.
5. Those innovations that did not contradict the Shari’ah in any form or fashion were allowed, not forbidden.
6. When confronted with any practice seemingly new, we should ask ourselves “how does it contradict the Shari’ah?”
 Saheeh al-Bukhaari, English translation, vol. 6, hadeeth no. 509
 There is no dispute in the fact that there are twenty units for the Taraaweeh, for Imaam Muwaffaq ud-Deen has pointed out in his al-Mughni that the Sahaabah are in consensus about it. Those who go against the Ijmaa’a are not considered to be from the Ahl us-Sunnah wal-Jamaa’ah as they violate the consensus and rebel against the Khalifah ‘Umar (رضي الله عنه)
 Saheeh al-Bukhaari, English Translation, vol. 3, hadeeth no. 227. Those who opine that the hadeeth “every bid’ah is misguidance” - kulla bid’ah dalaalah – is in its most absolute sense argue that ‘Umar’s [radiyAllahu anhu] statement “ni’imatul bida’atu hadhihi - This is an excellent innovation” refers to a linguistic innovation and is not really an innovation due to the fact it has roots in the Shari’ah. Our answer to this: Firstly, this argument negates the “Kullu - every” because if “kullu – every” is to be understood in its most absolute sense then the Prophet sal Allahu ‘alayhi wasallam would have also been speaking about such “linguistic bid’ahs”. Secondly, this very same argument of “linguistic bid’ah” can be used to permit Milaad un-Nabi as this also has its origins in the Shari’ah and thus returns to it. The Salafis usage of “linguistic bid’ah” is therefore fallacious, nothing more than a semantic somersault in which the feet land on the very same surface they sprung from. For the traditional correct understanding of linguistic bid’ah refer to Imaam Ibn Rajab al-Hanbali’s commentary hadeeth no. 28 in his Jami’ al-‘Ulum wal-Hikaam, who gives a fair, just and balanced understanding of how linguistic bid’ah should be understood, especially given the fact he quotes from the sources that some misguided Muslims would have us believe to be fabricated, etc.
 Iqtidaa al-Siraat ul-Mustaqeem, pgs. 294 – 297
 See Ibn Rajab al-Hanbali’s Lata’if al-Ma’arif, pp158-216
 Surat ul-Hadeed (57) ayah 27
 See Tanweer ul-Miqbaas [Tafseer Ibn Abbas]