That the Messenger of Allah (ﷺ) came across 'Umar while he was on his mount, and he was swearing by his father. So the Messenger of Allah (ﷺ) said: "Verily Allah prohibits you from swearing by your fathers. So let the one who swears, swear by Allah, or be silent."
Jami` at-Tirmidhi Book 20, Hadith 12