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."
أَنَّ رَسُولَ اللَّهِ ﷺ أَدْرَكَ عُمَرَ وَهُوَ فِي رَكْبٍ وَهُوَ يَحْلِفُ بِأَبِيهِ فَقَالَ رَسُولُ اللَّهِ ﷺ إِنَّ اللَّهَ يَنْهَاكُمْ أَنْ تَحْلِفُوا بِآبَائِكُمْ لِيَحْلِفْ حَالِفٌ بِاللَّهِ أَوْ لِيَسْكُتْ