If he builds upon certainty, then he performs the two prostrations before the salam, not after the salam, contrary to the statement of those who claim that the two prostrations of forgetfulness are, in all cases, to be performed after the salam.

Sayyiduna Abu Sa'id Khudri رضي الله عنه narrates from the Messenger of Allah صلى الله عليه وسلم that he صلى الله عليه وسلم said: "When any one of you is uncertain in his prayer and does not know how many rak'ahs he has prayed—one, two, three, or four—then he should complete the number of rak'ahs about which he is in doubt, then while sitting in tashahhud, he should perform two prostrations. Thus, if his prayer was incomplete, he has now completed it, and the two prostrations will be a means of humiliation and disgrace for Satan. And if he had already completed his prayer, then this (extra) rak'ah and the two prostrations will be counted as voluntary (nafl) for him."