Sayyiduna Abu Sa'id Khudri (may Allah be pleased with him) narrates this statement of the Noble Prophet (peace and blessings be upon him): "When a person has doubt about his prayer and does not know how many rak'ahs he has performed: four or three? Then he should put aside the doubt and base it on what is certain, then stand up and recite (Qira'at) and perform one rak'ah. Then when he is sitting (i.e., reciting Tashahhud), before saying the salam, he should perform two prostrations of forgetfulness (sajda sahw). If he had already performed four rak'ahs, then now one rak'ah will be extra for him, and these two prostrations will make it even, which was the fifth rak'ah. And if he had already performed three rak'ahs, then the fourth rak'ah will complete it for him, and the two prostrations of forgetfulness will rub the nose of Satan in the dust."