Is there a significant disagreement among scholars regarding the permissibility of praying behind an innovator whose innovation does not expel him from the fold of Islam? And is it permissible to refute those who say it is permissible?

Those who adhere to innovations that lead to disbelief (Bid'ah Mukaffirah): some of them are declared disbelievers individually (kafir 'aynan) if they are not excused, while others are not declared disbelievers due to the absence of proof (Hujjah) against them.

If the person adhering to the innovation is deemed an individual disbeliever, then prayer behind him is not permissible by scholarly consensus.

If he is not deemed a disbeliever due to ignorance or misinterpretation (ta'wil), then prayer behind him is a matter of dispute. The more preponderant view is that he does not become a disbeliever. However, it is more appropriate to refrain from praying behind him if it is possible to pray behind someone else, as a deterrent and a form of boycotting his innovation. But if no one else is available, such as during the Eid prayers, then prayer behind him is permissible.

The Permanent Committee was asked: "Whoever finds an imam who is not an innovator should pray behind him, not behind an innovator. If his innovation is polytheistic (shirkiyyah), such as one who seeks help from the dead or invokes them besides Allah, then one should not pray behind him, because he is a disbeliever. If his innovation is not one that leads to disbelief, such as verbalizing the intention, then his prayer and the prayer of those behind him are valid."

Shaykh al-Islam Ibn Taymiyyah said: "If you do not find an imam other than him, such as for Friday prayers and Eid prayers, or for Hajj prayers behind the imam of the season: these are performed behind every righteous and wicked person, by agreement of the Ahl al-Sunnah wa al-Jama'ah." "However, if it is possible for him to pray behind someone who is not an innovator, that is undoubtedly better and more virtuous. But if he prays behind him, there is a dispute among scholars regarding his prayer."

In summary: Whoever adheres to an innovation that leads to disbelief, such as seeking aid from the dead, if the proof (Hujjah) has been established against him and he is not ignorant or misinterpreting: he is a disbeliever, and prayer behind him is not valid. However, if he is ignorant or misinterpreting and the proof has not been established against him: he is not deemed a disbeliever, but prayer behind him should be avoided as a deterrent and a form of boycotting him.