What is the ruling on one who prayed behind a disbeliever out of ignorance that it is impermissible to pray behind him, and is his prayer invalid or valid?

The imamate of a disbeliever is not valid. Therefore, whoever prays behind him, their prayer is invalid and must be repeated according to the majority of scholars, even if the worshipper was unaware of his disbelief. However, some scholars hold that repetition is not required for one who prayed behind him while ignorant of his disbelief. Nevertheless, prayer behind a disbeliever is never valid under any circumstances, and whoever prays behind him must repeat their prayer.