Musnad Ahmad 10:34

Muhammad ibn Yazid narrated to us, from Isma'il ibn Muslim, from al-Zuhri, from 'Ubayd Allah ibn 'Abd Allah, from Ibn 'Abbas, that he was discussing the matter of prayer with 'Umar. Then 'Abd al-Rahman ibn 'Awf came to them and said: "Shall I not tell you a hadith I heard from the Messenger of Allah, peace and blessings of Allah be upon him?" They said: "Yes." He said: "I bear witness that I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'Whoever performs a prayer and doubts about a deficiency, let him pray until he doubts about an excess.'"