What is the ruling on the prayer of one who recited the Tashahhud in the third rak'ah of the Asr prayer, then stood for the fourth rak'ah and performed two prostrations of forgetfulness (sujud al-sahw) before the salutation (taslim)?

It is not permissible for the ma'mum (follower in prayer) to continue following the imam (leader of prayer) if he knows that the imam has added to the prayer. They must alert him by saying "SubhanAllah" (Glory be to Allah). And whoever sits with him, knowing that it is forbidden, their prayer is nullified. As for the imam, his doubt about the fifth rak'ah (unit of prayer) does not affect the validity of his prayer, as long as he has performed four rak'ahs and performed the prostration of forgetfulness (sujud as-sahw).