Musnad Ahmad 2:159

'Abd al-Razzaq narrated to us, Isra'il informed us, from Simak, from 'Ikrimah, from Ibn 'Abbas, who said: 'Umar said: I was in a caravan traveling on a military expedition with the Prophet, may Allah bless him and grant him peace. I swore an oath and said, "No, by my father." A man from behind me rebuked me and said, "Do not swear by your fathers." I turned around, and behold, it was the Messenger of Allah, may Allah bless him and grant him peace.