It is narrated from Sayyiduna Abu Hurairah, may Allah be pleased with him: The Messenger of Allah, peace and blessings be upon him, said: "When the call to prayer (adhan) is given, Satan turns his back and flees while passing wind so that he does not hear the adhan. When the adhan is finished, he returns, and when the iqamah is pronounced, he flees again. When the iqamah is finished, he comes back and continues to cast doubts into the heart of a person, saying, 'Remember such and such, remember such and such,' reminding him of things he did not recall before, to the extent that the person does not remember how many units (rak‘ahs) he has prayed. So if any of you does not remember whether he has prayed three or four rak‘ahs, he should perform two prostrations while sitting (i.e., perform the prostration of forgetfulness—sajdah sahw)."

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.