The Ruling on Following an Innovator in Prayer

If a person’s prayer is valid for himself, is it valid for others, even if he affirms innovations?
Answer: Yes, if the innovation does not make him a disbeliever. If the innovation is considered a sin but not major shirk (associating partners with Allah), then one may pray behind him. However, if his innovation makes him a disbeliever, such as one who worships the dead or calls upon the dead for aid, this is called an innovation and is major shirk, so one does not pray behind him. Likewise, if his innovation entails disbelief, such as the innovation of the people of the Oneness of Existence who say: there is no worshipper and no worshipped, but rather the very essence of the worshipped God is the very essence of the created being. This is major disbelief, we seek refuge in Allah.
The conclusion is that for innovations that do not entail disbelief, one may pray behind their practitioner. It is good to boycott him until he repents; it is good that he is boycotted and one prays behind someone else until he repents, perhaps he will repent. This is fine. But if one prays behind him while he is not a disbeliever, the prayer is valid, just as one prays behind a sinner. This is according to the most correct opinion of the scholars, may Allah have mercy on them. Some of the Companions, such as Ibn 'Umar (may Allah be pleased with him) , prayed behind al-Hajjaj ibn Yusuf al-Thaqafi, the tyrant, who was among the most unjust and most sinful of people. And a large number of the righteous Salaf prayed behind him during Hajj, so there is no harm. Yes.