What is the ruling on the prayer of one who says in the Tashahhud: "I bear witness that there is no god but Allah and I bear witness that Muhammad is His slave and Messenger, and that Jesus, Moses, and Idris are Allah's prophets and messengers, and that Gabriel is one of the angels"?

This addition in the Tashahhud is not legislated and is considered an innovation (bid'ah); due to its not being reported from the Prophet (peace be upon him) or his Companions. Nevertheless, the prayer is not invalidated by it because it is part of the prescribed remembrance (dhikr), and the jurists have stipulated that prayer is not invalidated by dhikr and supplication (du'a), even if they are not specifically recommended within it.