Musnad Abu Ya'la al-Mawsili 7:22

Al-Hasan ibn ‘Umar ibn Shaqiq ibn Asma’ al-Jarmi narrated to us, my father narrated to us, Isma’il narrated to us, from al-Zuhri, from ‘Ubayd Allah ibn ‘Abd Allah ibn ‘Utbah, from Ibn ‘Abbas, who said: He and ‘Umar were discussing prayer. He said: Then ‘Abd al-Rahman ibn ‘Awf passed by us and said: Shall I not narrate to you a hadith I heard from the Messenger of Allah, may Allah bless him and grant him peace? He said: So I bear witness with the testimony of Allah that I heard the Messenger of Allah, may Allah bless him and grant him peace, saying: "If one of you prays and is in doubt regarding a deficiency in his prayer, let him pray until he is in doubt regarding an excess."