Musnad Ahmad 2:133

Muhammad ibn Abdullah al-Zubayri narrated to us, Isra'il narrated to us, from Simak, from Ikrimah, from Ibn Abbas, from Umar, who said: I was with the Prophet, peace and blessings of Allah be upon him, on a military expedition, and I swore, "No, by my father." A man called out to me from behind me and said, "Do not swear by your fathers." And behold, it was the Prophet, peace and blessings of Allah be upon him.