Musnad Ahmad 2:160

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from al-Zuhri, from Salim, from his father, from 'Umar, who said: The Messenger of Allah (ﷺ) heard me while I was swearing by my father, so he said: "Indeed, Allah forbids you from swearing by your fathers." 'Umar said: By Allah, I have not sworn by them since then, neither as a narrator nor as one repeating it.