Musnad Ahmad 2:35

Abu Sa’id, the freed slave of Banu Hashim, narrated to us, saying: Za’idah narrated to us, Simak narrated to us, from ‘Ikrimah, from Ibn ‘Abbas, who said: ‘Umar said: We were with the Messenger of Allah, may Allah bless him and grant him peace, in a traveling party, and a man said: "No, and by my father." A man said: "Do not swear by your fathers." He turned around, and behold, it was the Messenger of Allah, may Allah bless him and grant him peace.