From his father (Ibn 'Umar) that the Prophet (ﷺ) heard 'Umar saying: "By my father, By my father!" So he said: "Verily Allah prohibits you from swearing by your father." So 'Umar said: "By Allah I did not swear by him after that, neither intentionally nor in narrating."
Jami` at-Tirmidhi Book 20, Hadith 11
← Previous Hadith That the Messenger of Allah (ﷺ) said: "Whoever swears and says: 'If Allah wills (Insha Allah), then he will not have broken it."

Jami` at-Tirmidhi Book 20, Hadith 10 Next Hadith → 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