Musnad Ahmad 1656

It was narrated from Ibn "Abbas (رضي الله عنه) that "Umar (رضي الله عنه) said to him: O boy, did you hear from the Messenger of Allah (ﷺ) or from one of his Companions anything about what a man should do if he is not sure about his prayer? Whilst he was saying that, ‘Abdur-Rahman bin "Awf came and said: What are you talking about? "Umar said: I asked this boy whether he heard anything from the Messenger of Allah (ﷺ) or from one of his Companions about what a man should do if he is not sure about his prayer. ‘Abdur-Rahman said: I heard the Messenger of Allah (ﷺ) say: “If one of you is not sure about his prayer and does not know whether he prayed one or two [rak"ahs], let him assume it is one: and if he does not know whether he prayed two or three, let him assume that it is two; and if he does not know whether he prayed three or four, let him assume that it is three; then let him prostrate twice when he finishes his prayer, whilst he is still sitting, before he says the salam."