Ibn Hibban

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 has doubt about his prayer, he should put aside the doubt and base his action on what he is certain of. If he is certain that his prayer is complete, then he should perform two prostrations of forgetfulness (sajda sahw). If his prayer was complete, then that rak‘ah will become nafl (supererogatory) and the two prostrations will also be nafl. And if his prayer was incomplete, then that rak‘ah will complete his prayer and these two prostrations will rub the nose of Satan in the dust." (Imam Ibn Hibban, may Allah have mercy on him, says:) "A person who does not have expertise in the science of hadith and does not understand authentic narrations has fallen into the misunderstanding that taharri (making an assumption) and basing action on certainty are the same thing regarding prayer, whereas this is not the case. Taharri is when a person has doubt in his prayer and does not know how much he has prayed. When such a situation arises, it is necessary for him to make an assumption about the correct situation, which is most likely in his view. He should act upon that and after saying salam, perform two prostrations of forgetfulness, as is mentioned in the narration from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him). And the case of basing action on certainty is when a person has doubt between two or three rak‘ahs, or three or four rak‘ahs. So when such a situation arises, he should act upon certainty, which is the lesser number, then complete his prayer and before saying salam, perform two prostrations of forgetfulness, as is mentioned in the narration from Sayyiduna Abdur Rahman bin Awf (may Allah be pleased with him) and Sayyiduna Abu Sa'id Khudri (may Allah be pleased with him). Both methods are Sunnah, and there is no contradiction between the two."