Musnad Ahmad 2:31

Bishr ibn Shu'ayb ibn Abi Hamzah narrated to us, he said: My father narrated to me, from al-Zuhri, he said: Salim ibn Abdullah informed me that Abdullah ibn Umar informed him that Umar ibn al-Khattab said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "Indeed, Allah, the Almighty and Majestic, forbids you from swearing by your fathers." Umar said: By Allah, I have not sworn by them since I heard the Messenger of Allah, peace and blessings of Allah be upon him, forbid it, nor have I spoken them, neither as a speaker nor as a narrator.