Musnad Ahmad 2:210

Husayn ibn Muhammad narrated to us, Israel narrated to us, from Simak, from Ikrimah, from Ibn Abbas, who said: Umar said: I was in a traveling party, journeying on a military expedition with the Messenger of Allah, may Allah bless him and grant him peace. I swore an oath and said, "No, and by my father." A man called out to me from behind me, "Do not swear by your fathers." I turned around, and it was the Messenger of Allah, may Allah bless him and grant him peace.