All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
As for the Imaam who missed a prostration due to forgetfulness and did not go back to perform it due to this forgetfulness and not comprehending the Tasbeeh of the followers and then, after the prayer ended, he prayed a Rak‘ah without a long interval between this prayer and the Tasleem, then his prayer is valid and he has to perform the prostration of forgetfulness.
This is regarding the ruling on his prayer. Those who pray behind him must notify him and, if he does not go back, they have to perform the prostration that he missed and not follow him in leaving it and then catch up with him to complete the prayer. If the Imaam stands to make up the missed Rak‘ah, they should not follow him because their prayer is complete. If the Imaam observed this defect before making Tasleem and stood to perform a Rak‘ah and they followed him knowingly, then their prayer will be invalid because they intentionally performed an additional Rak‘ah.
Allaah Knows best.