Sayyiduna Abu Sa'id Khudri (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) said: "When any one of you does not know whether he has prayed three rak'ahs or four, he should stand up and pray one more rak'ah, then perform two prostrations of forgetfulness (sujood as-sahw). If that rak'ah turns out to be the fifth, then these prostrations will make his prayer even; and if it is the fourth rak'ah, then these two prostrations will humiliate Satan." Imam Darimi (may Allah have mercy on him) said: I am one who holds this view and acts upon it.