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." (Using translation from Tirmidhī 1534)
أَنَّ رَسُولَ اللهِ ﷺ أَدْرَكَ عُمَرَ فِي رَكْبٍ يَحْلِفُ بِأَبِيهِ فَقَالَ «إِنَّ اللهَ يَنْهَاكُمْ أَنْ تَحْلِفُوا بِآبَائِكُمْ حَالِفٌ يَحْلِفُ بِاللهِ أَوْ لِيَسْكُتْ»