When Abū ʿAmr ʿUthmān ibn Marzūq arrived in Egypt, its rulers at that time openly professed Shiism and were heretical Bātinīs (esotericists). As a result, religious innovations had proliferated and become widespread in Egypt. He therefore instructed his followers to pray only behind those they knew. After his death, Sunni rulers such as Saladin conquered Egypt, and the teachings of the Sunnah, opposed to those of the Rāfidah, gained prominence there. Knowledge and the Sunnah then became increasingly widespread and manifest.
Praying behind someone whose inward condition is unknown is therefore permissible by agreement of the Muslim scholars. Whoever says that prayer behind someone whose condition is unknown is forbidden or invalid has opposed the consensus of the people of the Sunnah and the community.
The Companions, may Allah be pleased with them, used to pray behind people whose sinful conduct they knew. Thus, ʿAbd Allāh ibn Masʿūd and other Companions prayed behind al-Walīd ibn ʿUqbah ibn Abī Muʿayt, who drank wine and once prayed four units for the dawn prayer. ʿUthmān ibn ʿAffān had him flogged for this. ʿAbd Allāh ibn ʿUmar and other Companions also prayed behind al-Hajjāj ibn Yūsuf. The Companions and the Successors prayed behind Ibn Abī ʿUbayd, who was suspected of heresy and called people to misguidance.