Definition of bid’ah.
Shaykh Muhammad ibn ‘Uthaymeen (may Allaah have mercy on him) said: “According to sharee’ah, the definition is ‘Worshipping Allaah in ways that Allaah has not prescribed.’ If you wish you may say, ‘Worshipping Allaah in ways that are not those of the Prophet (peace and blessings of Allaah be upon him) or his rightly guided successors (al-khulafaa’ al-raashidoon).’”
The first definition is taken from the aayah (interpretation of the meaning):
“Or have they partners with Allaah (false gods) who have instituted for them a religion which Allaah has not ordained?”
The second definition is taken from the hadeeth of the Prophet (peace and blessings of Allaah be upon him), who said:
“I urge you to adhere to my way (Sunnah) and the way of the rightly-guided successors (alkhulafa’ al-raashidoon) who come after me. Hold fast to it and bite onto it with your eyeteeth [i.e., cling firmly to it], and beware of newly-invented matters.”
So everyone who worships Allaah in a manner that Allaah has not prescribed or in a manner that is not in accordance with the way of the Prophet (peace and blessings of Allaah be upon him) or his rightly-guided successors (al-khulafa’ al-raashidoon), is an innovator, whether that innovated worship has to do with the names and attributes of Allaah, or to do with His rulings and laws.
With regard to ordinary matters of habit and custom, these are not called bid’ah (innovation) in Islam, even though they may be described as such in linguistic terms. But they are not innovations in the religious sense, and these are not the things that the Prophet (peace and blessings of Allaah be upon him) was warning us against. And there is no such thing in Islam as bid’ah hasanah (good innovation).”
(Majmoo’ Fataawa Ibn ‘Uthaymeen, vol. 2, p. 291)