Musnad Ahmad 27:101

Sufyan narrated to us, from al-Zuhri, who heard Salim, from his father, that the Messenger of Allah (ﷺ) heard Umar (may Allah be pleased with him) saying: "And by my father, and by my father." He said: "Indeed, Allah the Almighty forbids you from swearing by your fathers." Umar said: "By Allah, I have not sworn by them since, neither as one mentioning it nor as one reporting it."