Definition of bid'ah (innovation in religion)
Although the definition of bid'ah is simple and straight forward, people are often confused and are unable to differentiate between sunnah (practice of the Prophet), bid'ah (innovation in religion) and mub'ah (neutral non-religious actions). Simply stated, anything added (increased) in religion or in the name of religion is bid'ah irrespective of the motive or reason for the addition, even if the intention is that of extreme love and devotion.
و بهذا الإسناد عن أحمد بن أبي عبد الله عن عبد الله بن محمد الحجال عن عاصم بن حميد رفعه قال جاء رجل إلى أمير المؤمنين ع فقال أخبرني عن السنة و البدعة و عن الجماعة و عن الفرقة فقال أمير المؤمنين ع السنة ما سن رسول الله ص و البدعة ما أحدث من بعده و الجماعة أهل الحق و إن كانوا قليلا و الفرقة أهل الباطل و إن كانوا كثيرا
A man came to Amir al-Mumineen (عليه السلام), and said: Inform me about al-sunnah, al-bid'ah, al-jama'ah, and al-firqah. So Amir al-Mumineen (عليه السلام) said: al-sunnah is whatever the Messenger of Allah (s) performed, al-bid'ah is whatever has been innovated after him (s), al-jama'ah are the people of truth, even if they are little, and al-firqah are the people of falsehood even if they are a lot.
[al-Sadooq, Ma`aani al-Akhbaar (Qom: Inshārāt Islāmī, 1361), pg. 154 - 155, hadeeth # 3]
Domain of bid'ah
It is important to understand that bid'ah pertains only to religious actions. By religious actions we mean those actions that define an individual's religious identity. These are the actions that are performed in the name of religion and are connected one way or another to the religious choice of the individual. All other actions can be termed as non-religious and they may be classified as mub'ah - being neither good nor bad from a religious point of view and which merit neither reward nor punishment.
These are the actions that don't define the religious identity and are usually practiced by people across all religions. Some examples are travelling by a car, wearing a cultural dress, speaking a particular language and following a regional diet. None of these define the religious identity of the individual. The distinction between religious and non-religious actions is important otherwise all the modern inventions and cultural practices would (God forbid!) come under bid'ah - since they were not practiced by the Holy Prophet (s).
Clarification on some confusing actions
Certain religious actions are performed using means that did not exist during the time of the Prophet (s). For example praying on a carpet, using a loudspeaker for adhaan or playing audio recording of a dua. Are these bid'ah? It depends on the intention of the performer and which aspect of the action he/she considers as part of religion. If the carpet or loudspeaker is considered as an obligatory or recommended part of religion then it is a bid'ah. If the performer simply uses it out of convenience and understands that its use itself is not a part of religion then it is not a bid'ah. Carpets, loudspeakers and audio technology are also used by non-Muslims for other purposes so they don't define a religious identity and therefore cannot be bid'ah if used with the intention of convenience.
Bid'at-ul-hasanah (Good Bid'ah)
In the school of Ahlul Bayt (عليه السلام), there is no concept of bid'at-ul-hasanah (good innovation). This concept was invented by the Ahlul Sunnah in order to justify the bid'ah of their caliphs. For example, after establishing the Tarawih prayers in the month of Ramadhan, Umar bin Khattab remarked: "How good this bid'ah is!". The scholars of Ahlul Sunnah have confused the people by complicating the simple definition of bid'ah, created different categories of bid'ah, and wrote lengthy tortuous explanations using reason and logic. See for example: The Concept of Bid'ah in the Shari'ah
Some people argue that if a new practice is established in the name of religion and there are no express statements of Ahlul Bayt (عليه السلام) forbidding it then it is not a bid'ah. This line of argument is the same as those who practice bid'at-al-hasanah.
Actions should be based only on Sunnah
وعن عدة من أصحابنا، عن أحمد بن محمد بن خالد، عن أبيه، عن أبي إسماعيل إبراهيم بن إسحاق الأزدي، عن أبي عثمان العبدي، عن جعفر، عن آبائه، عن أمير المؤمنين ( عليه السلام ) قال: قال رسول الله ( صلى الله عليه وآله ): لا قول إلا بعمل، ولا قول و لا عمل إلا بنية، ولا قول و لا عمل و لا نية إلا بإصابة السنة. ورواه الشيخ مرسلا عن الرضا ( عليه السلام )، نحوه. ورواه المفيد في ( المقنعة ) مرسلا. ورواه البرقي في ( المحاسن ) عن أبيه، بالإسناد.
The Messenger of Allah (سُبْحَانَهُ وَ تَعَالَى) said: There is no saying but through action (la qawl illa bi amal), and there is no saying and no action but by intention (la qawl va la amal illa bi niyyah), and there is no saying and no action and no intention but by achieving the Sunnah (la qawl wa la amal va la niyyah illa b'isabatil sunnah).
[Al-Kafi, Vol. 1, H 207, Ch. 22, h9]
According to the above hadith every saying, action and intention should be based on sunnah. Whatever is not done to fulfill the sunnah has no value and no merit in the eyes of Allah (سُبْحَانَهُ وَ تَعَالَى).
Better to have fewer actions
Nowadays, adherants of the Usuli school are steeped in innovations. They are worried that if all the practices are weakened there will not be much to do. The following hadith should be an eye-opener for them:
The Holy Prophet (s) said: Few actions in sunnah are better than many in bid'ah.
[Al-Tūsī, Tahdhīb al-Aḥkām, 10 vols., (Tehran: Dār al-Kutub al-Islāmiyyah, 4th Edition, 1407 AH), vol. 3, pg. 70, hadeeth # 29]
Duty of the scholars in the face of bid'ah
"When innovations emerge it is obligatory for the scholars to make their knowledge public (I.e. expose the bid'ah), otherwise, Allah will send la'anat (condemn) on them."
[Al Kafi Chapter: 19, Hadith: 157, Number: 2]
(Also published on blog: https://ahlulbaytmission.org/2019/08/22/definition-and-scope-of-bidah)