Sayyiduna Abu Sa'id Khudri (may Allah be pleased with him) narrates that the Prophet (peace and blessings of Allah be upon him) said: "When a person offers prayer and is unsure whether he has performed three rak'ahs or four, then while he is sitting, he should perform two prostrations of forgetfulness, because Satan comes to a person and says, 'You have become impure (lost your wudu),' so that person should say, 'You are lying.' However, if he hears a sound (of passing wind) with his ears or perceives a smell with his nose, (then the ruling is different)."