Shaykh Muhammad ibn ‘Uthaymeen (may Allah have mercy on him) said: “According to shari’ah, the definition [of bid'aah] is ‘Worshipping Allah in ways that Allah has not prescribed.’ If you wish you may say, ‘Worshipping Allah in ways that are not those of the Prophet (peace and blessings of Allah be upon him) or his rightly guided successors (al-khulafaa’ al-raashidoon).’”
A few resources I came across that I am looking at for creating lessons on the topic of bid'ah, in shaa Allah.
(They are roughly the same in content and layout but there were some differences that I thought I'd like to pull from each of them)
- Categories of Bidah - Al Ifta.net
- How Bid'ah Takes Away the Imaan
- Do Not Compromise the Religion Ever and Beware of Tricks of Ahlul Bid'ah (this is a good one!)
- Avoiding Bid’ah Involves Knowing Six Affairs Ibn al Uthaymeen by Abu Khadeejah (Audio)
- Innovation in Light of Perfection of the Shariah
- AbdurRahman.org's Innovation Tag- lots of treatises pertaining to the subject
Bid'aah - Don't Do it, Forbid it, & Warn Against It - from Calgary Islam (this link no longer works but I thought the excerpt was worth leaving up.
Excerpt:
...The Prophet (sallAllaahu 'alayhi wa sallam) also advised us, "I warn you of the newly invented matters (in the religion), and every newly invented matter is an innovation, and every innovation is misguidance, and every misguidance is in the Hellfire"
Some ask, "What's the big deal? We have bigger problems to tackle." Some claim it's between them and Allaah ('azza wa jall). And some claim their intention is "good."
To those who stated there are bigger problems, please understand there is no bigger problem than failing to protect the purity of Islam and failing to stop the division we suffer from. What bigger problem could there possibly be than the dilution of Islam and the deterioration of the deen Allaah chose and perfected for us? And these are just a few of the consequences of bid'ah, as you'll soon read, inshaAllaah, along with their proofs.