The Prophet ﷺ said, "If anybody has to take an oath, he should swear only by Allah." The people of Quraish used to swear by their fathers, but the Prophet ﷺ said, "Do not swear by your fathers. " (Using translation from Bukhārī 3836)  

قَالَ رَسُولُ اللهِ ﷺ مَنْ كَانَ حَالِفًا فَلَا يَحْلِفُ إِلَّا بِاللهِ وَكَانَتْ قُرَيْشٌ تَحْلِفُ بِآبَائِهَا فَقَالَ «لَا تَحْلِفُوا بِآبَائِكُمْ»  

"A man from Banu Ghifar told me, in the gathering of Salim bin 'Abdullah, Salim bin 'Abdullah said: 'I heard 'Abdullah -that is, Ibn 'Umar- say: "The Messenger of Allah said: 'Allah forbids you to swear by your forefathers.'"'" (Using translation from Nasāʾī 3765)  

قَالَ رَسُولُ اللهِ ﷺ «إِنَّ اللهَ يَنْهَاكُمْ أَنْ تَحْلِفُوا بِآبَائِكُمْ»