Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "When any of you is in doubt in his prayer (about how many rak'ahs he has prayed), let him discard the doubt and build upon what is certain (that is, the lesser number). Then when he is certain of completion, let him make two prostrations. If his prayer was complete, that one rak'ah and the two prostrations will be voluntary; and if his prayer was deficient, that rak'ah will complete his prayer, and the two prostrations will be a cause of humiliation for Satan." This hadith is sahih (authentic) on the condition of Imam Muslim, but they did not record it with this wording!

Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrates that the Messenger of Allah (peace and blessings be upon him) said: "When any of you prays and does not know how many rak'ahs he has prayed, three or four, let him pray one (more) rak'ah, performing its bowing and prostration well, then make two prostrations (of forgetfulness)." This hadith is sahih (authentic) on the condition of the two Shaykhs (Bukhari and Muslim), but they did not record it.