"Allah forbids you to swear by your forefathers." 'Umar said: "By Allah, I never swore by them again, whether saying it for myself or reporting of others." (Using translation from Nasāʾī 3767)
أَنَّ رَسُولَ اللهِ ﷺ قَالَ «إِنَّ اللهَ يَنْهَاكُمْ أَنْ تَحْلِفُوا بِآبَائِكُمْ» قَالَ عُمَرُ فَوَاللهِ مَا حَلَفْتُ بِهَا بَعْدُ ذَاكِرًا وَلَا آثِرًا