What is the ruling on innovation (bidʻah) and the innovator (mubtadiʻ)?

Innovation in religion (bid‘ah) is forbidden, based on the Prophet’s (peace be upon him) saying: “Every innovation is a misguidance.” The repentance of an innovator is not accepted until he abandons his innovation, according to the hadith: “Indeed, Allah has withheld repentance from every innovator until he abandons his innovation.” Dealing with an innovator, praying behind him, and shunning him varies depending on whether the innovation is one that constitutes disbelief or not.