Is it permissible to pray behind people of innovation, such as the Sufis?

One must seek an Imam who is not an innovator (a person who introduces Bid'ah). If no one is available except an innovator, then he should be advised and the truth explained to him. If his innovation is of a nature that constitutes disbelief (Kufr), such as seeking help from other than Allah, or supplicating to other than Him, or sacrificing to other than Him, or slandering Aisha, or believing the Quran to be incomplete, then prayer behind him is invalid, and he is not permitted to be an Imam. However, if his innovation does not constitute disbelief, then prayer behind him is valid.