Musnad Abu Ya'la al-Mawsili 7:5

Abu Khaythama narrated to us, Ya'qub ibn Ibrahim narrated to us, my father narrated to us, from Ibn Ishaq, who said: Makhul narrated to me, from Kurayb, the freed slave of Ibn Abbas, from Abdullah ibn Abbas, who said: I sat with Umar ibn al-Khattab, and he said to me: "O Ibn Abbas, have you heard anything from the Messenger of Allah (may Allah bless him and grant him peace) that he commanded a Muslim man to do if he forgets in his prayer, how he should act?" I said: "No, by Allah. Or have you not heard anything from the Messenger of Allah (may Allah bless him and grant him peace) regarding that, O Commander of the Faithful?" He said: "No, by Allah." While we were in that state, Abd al-Rahman ibn Awf came and said: "What are you two discussing?" Umar said to him: "I asked him," and he informed him of what he had asked him. Abd al-Rahman said to him: "But I have heard the Messenger of Allah (may Allah bless him and grant him peace) command regarding that." Umar said to him: "You are trustworthy to us, so what did you hear from the Messenger of Allah (may Allah bless him and grant him peace)?" Abd al-Rahman said: "I heard the Messenger of Allah (may Allah bless him and grant him peace) say: 'If one of you forgets in his prayer until he does not know whether he added or subtracted, if he is in doubt between one and two, let him count it as one. If he is in doubt between two and three, let him count it as two. And if he is in doubt between three and four, let him count it as three, so that the uncertainty is regarding the addition. Then he should prostrate twice while he is sitting before he gives the salutation, then he should give the salutation.'"