Innovation is a newly invented worship which was not revealed in the Shari’ah (the Qur’an and Sunnah)―so it is referred to as bid’ah. And every bid’ah is misguidance due to the saying of the Prophet (salallāhu ‘alaihi wasallam): “Every newly introduced matter is a bid’ah, and every bid’ah is misguidance.” And he (salallāhu ‘alaihi wasallam) used to say in his khutbah: “Indeed the best speech is the Book of Allah and the best of guidance is the guidance of Muhammad, and the worst of all affairs are the newly introduced matter.” And he (salallāhu ‘alaihi wasallam) used to say: “Every innovation is misguidance.” And he (salallāhu ‘alaihi wasallam) would say: “Whoever introduces into this affair of ours (Islam) that which is not from it, then it will be rejected.” And he (salallāhu ‘alaihi wasallam) said: “Whoever does a deed that is not upon our affair (Islam) then it will be rejected.” So innovation is that which is newly introduced into the Religion from the matters of worship that have no basis (or proof) for them. All innovations are forbidden.
Those who divide bid’ah into five categories: obligatory (wājib), prohibited (harām), disliked (makrūh), recommended (mustahabb) and allowed (mubāh)―then this categorisation of theirs is false and not correct. What is correct is that all innovations are misguidance, just as the Prophet (salallāhu ‘alaihi wasallam) said.
Is it allowed to pray behind an innovator?
If his innovation is unbelief (mukaffirah), then it is not allowed to pray behind him, such as the bid’ah of the Jahmiyyah, Mu’tazilah and those who resemble them. However, if the bid’ah is not unbelief such as the one who raises his voice in making intention for the Prayer by uttering, “I intend to pray such-and-such salāh,” and other such innovations, then it is allowed to pray behind them. And he should be taught not to raise his voice, rather he must keep his intention in his heart, and that is sufficient. And likewise, in this category is the innovation of gathering for and celebrating birthdays as long as there is no shirk (polytheism) involved, this innovation is not unbelief. But if it involves supplicating to the Prophet (salallāhu ‘alaihi wasallam), or supplicating to someone else, or seeking rescue and deliverance from him, whether it be on the birthday of ‘Ali (radiyallāhu ‘anhu), or Hasan or Husayn (radiyallāhu ‘anhumā), or the Prophet (salallāhu ‘alaihi wasallam), or Fātimah (radiyallāhu ‘anhā) and supplication are made to them and they are called upon, or their aid is sought, or vows are made in their name, or animals are sacrificed to them―then this is major polytheism (shirk), and so there is no prayer behind them. But if they gather to read the Quran on such occasions, or to eat food―then this is bid’ah and the person has not committed unbelief (kufr).
Source: Nūr ‘alad-Darb, here.