I heard ʿUmar saying, "Messenger of Allah ﷺ said to me, 'Allah forbids you to swear by your fathers." ʿUmar said, "By Allah! Since I heard that from the Prophet ﷺ , I have not taken such an oath, neither intentionally, nor by reporting the oath of someone else."  

لِي رَسُولُ اللَّهِ ﷺ إِنَّ اللَّهَ يَنْهَاكُمْ أَنْ تَحْلِفُوا بِآبَائِكُمْ قَالَ عُمَرُ فَوَاللَّهِ مَا حَلَفْتُ بِهَا مُنْذُ سَمِعْتُ النَّبِيَّ ﷺ ذَاكِرًا وَلاَ آثِرًا قَالَ مُجَاهِدٌ {أَوْ أَثَرَةٍ مِنْ عِلْمٍ} يَأْثُرُ عِلْمًا  

that he found ʿUmar bin Al-Khattab in a group of people and he was swearing by his father. So Messenger of Allah ﷺ called them, saying, "Verily! Allah forbids you to swear by your fathers. If one has to take an oath, he should swear by Allah or otherwise keep quiet."  

أَنَّهُ أَدْرَكَ عُمَرَ بْنَ الْخَطَّابِ فِي رَكْبٍ وَهْوَ يَحْلِفُ بِأَبِيهِ فَنَادَاهُمْ رَسُولُ اللَّهِ ﷺ أَلاَ إِنَّ اللَّهَ يَنْهَاكُمْ أَنْ تَحْلِفُوا بِآبَائِكُمْ فَمَنْ كَانَ حَالِفًا فَلْيَحْلِفْ بِاللَّهِ وَإِلاَّ فَلْيَصْمُتْ