‘Ata b. Yasar said that Abu Sa'id reported God’s Messenger as saying,“When one of you is in doubt aoout his prayer and does not know how much he has prayed, three or four rak'as, he should subtract what he is doubtful about and base his prayer on what he is sure of, then perform two prostrations before giving the salutation. If he has prayed five rak'as they will make his prayer an even number for him, and if he has prayed exactly four they will be a humbling for the devil.”Muslim transmitted it, and Malik transmitted it from ‘Ata’ in mursal form. In his version it says,“he will make it an even number by these two prostrations.”