What is the ruling on praying behind an Imam who believes that the hijab is not obligatory, and what is the ruling on declaring him a disbeliever, and is it permissible to hold another prayer in the same mosque?

The hijab is an obligation explicitly evidenced by the Quran and Sunnah, and whoever denies it is in grave danger. The preacher who denied this must fear Allah and reconsider what he said. He should not be declared a disbeliever except after establishing proof and understanding the type of hijab he denies. If his intent is to deny the hijab in general, then his statement is disbelief, and proof must be established against him. Prayer behind someone who denies the obligation of the hijab, which the Ummah has unanimously agreed upon, is not valid for those who are aware of his stance.