Makhool said:

The Messenger of Allah (ﷺ) said: "If one of you prays and is uncertain about his prayer, if he is not sure whether it was one or two (rak"ahs), let him assume it was one; if he is not sure whether it was two or three, let him assume it was two; if he is not sure whether it was three or four, let him assume it was three, until he doubts only that he may have done too much, then let him prostrate twice before he says the salam, then let him say the salam." Muhammad bin Ishaq said: and Husain bin ‘Abdullah said to me; Did he tell you the isnad? I said: No. He said: But he told me that Kuraib, the freed slave of Ibn "Abbas, told him that Ibn "Abbas said: I sat with "Umar bin al-Khattab and he said: O Ibn "Abbas, if a man is not sure about his prayer, and does not know whether he prayed too many or too few [rak"ahs]. (what should he do)? I said: By Allah, O Ameer al-Mu"mineen, I do not know; I did not hear anything about that. "Umar said: By Allah, I do not know either. Whilst we were like that, ‘Abdur-Rahman bin ‘Awf came and said: What are you talking about?ʻUmar said to him: We were talking about a man who is not certain about his prayer; what should he do? He said: I heard the Messenger of Allah (ﷺ) say.... and he narrated the same Hadeeth.