بسم الله الرحمن الرحيم
Q: Is it permissible to pray behind an innovator, propagandist?
A: Prayer behind an innovator propagandist is not permissible from what is more acceptable from the verdicts of the scholars, unless he is the leader, in that case do not lag behind as it relates to congregational prayers (weekly and on Jumu’ah). So pray behind him, however if he is a Jahmi, or Shiite and you are forced to pray behind him, then repeat your prayer. This is Imaam Ahmad’s and those like him from the imaams of the Sunnah’s verdict.
Translated by Abu Abdil Waahid Najeeb Al Anjelesi
Source: فتاوى فضيلة الشيخ العلامة ربيع بن هادي المدخلي