Musnad Ahmad 10:22

Isma'il narrated to us, Muhammad ibn Ishaq narrated to us, Makhul narrated to me, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "If one of you prays and doubts in his prayer, if he doubts between one and two, let him consider them as one. If he doubts between two and three, let him consider them as two. If he doubts between three and four, let him consider them as three, so that the uncertainty is regarding the addition. Then let him prostrate two prostrations before he gives the salutation, then let him give the salutation." Muhammad ibn Ishaq said: Husayn ibn Abdullah said to me, "Did he attribute it to the Prophet for you?" I said, "No." He said, "But he narrated to me that Kurayb, the freed slave of Ibn Abbas, narrated to him from Ibn Abbas, who said: I sat with Umar ibn al-Khattab, and he said: 'O Ibn Abbas, if a man becomes confused in his prayer and does not know whether he added or subtracted?' I said: 'By Allah, O Commander of the Faithful, I do not know; I have not heard anything regarding that.' Umar said: 'By Allah, I do not know either.'" He said: While we were in that state, Abd al-Rahman ibn Awf came and said: "What is this that you two are discussing?" Umar said to him: "We mentioned the man who doubts in his prayer, how he should act." He said: "I heard the Messenger of Allah (peace and blessings of Allah be upon him) say this hadith."