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 is in doubt and does not know whether he has performed three rak'ahs or four, he should stand up and perform one rak'ah, completing its bowing (ruku') and prostration (sujud). Then, when he is sitting, he should perform two prostrations of forgetfulness (sajda sahw). If he had performed five rak'ahs, these two prostrations will make them even, and if he had already performed four rak'ahs, these two prostrations will disgrace Satan." (Imam Ibn Hibban, may Allah have mercy on him, says:) The narration transmitted from Sayyiduna Abdullah ibn Mas'ud (may Allah be pleased with him) and Sayyiduna Abu Sa'id Khudri (may Allah be pleased with him) is the narration that caused a learned person to fall into the misunderstanding that making an estimation (taharrī) during prayer and acting upon certainty (yaqīn) are the same thing. Whereas the rulings of both are different. In the narration transmitted from Sayyiduna Abdullah ibn Mas'ud (may Allah be pleased with him), there is the ruling of estimation (taharrī), in which the Prophet (peace and blessings of Allah be upon him) ordered to perform two prostrations of forgetfulness after the salām. Whereas in the narration transmitted from Sayyiduna Abu Sa'id Khudri (may Allah be pleased with him), there is the ruling of acting upon certainty (yaqīn), in which the Prophet (peace and blessings of Allah be upon him) ordered to perform the prostrations of forgetfulness before the salām. So, there is a difference between making an estimation (taharrī) and acting upon certainty (yaqīn). Because acting upon certainty means that if a person is in doubt about his prayer and does not know whether he has performed three rak'ahs or four, when such a situation arises, he will act upon what he is certain of, and that is three rak'ahs. Then he will complete his prayer and perform two prostrations of forgetfulness before the salām. As for estimation (taharrī), its situation is that a person starts the prayer, then his mind becomes occupied with some worldly or religious matter to the extent that he does not know how much of the prayer he has performed. When such a situation arises, he will make an estimation according to what is most likely, and whatever is established for him as a result of this estimation, he will act upon it, then complete his prayer and perform two prostrations of forgetfulness after the salām. In this way, both narrations can be acted upon together.