Musnad Ahmad 482:114

Abu Abd al-Rahman narrated to us, Haywah narrated to us, Bakr ibn Amr narrated to us, that Mushrah ibn Ha'an informed him that he heard Uqbah ibn Amir saying: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "If there were to be a prophet after me, it would have been Umar ibn al-Khattab."