Sayyiduna Abu Sa'id Khudri (may Allah be pleased with him) narrates this statement of the Noble Prophet (peace and blessings of Allah be upon him): "When a person performs prayer and does not know whether he has performed three rak'ahs or four rak'ahs, then he should perform one more rak'ah and before saying the salam, he should perform the prostration of forgetfulness (sajdah sahw) twice. If he had performed three rak'ahs, then these two prostrations will make them even, and if he had performed four, then these two prostrations will disgrace Satan." (Imam Ibn Hibban, may Allah have mercy on him, says:) This narration has been transmitted by Imam Ahmad ibn Hanbal from Safwan bin Salih.