He who has to take an oath, he must not take oath but by Allah. The Quraish used to take oath by their fathers. So he (the Holy Prophet) said: Do not take oath by your fathers.
قَالَ رَسُولُ اللَّهِ ﷺ مَنْ كَانَ حَالِفًا فَلاَ يَحْلِفْ إِلاَّ بِاللَّهِ وَكَانَتْ قُرَيْشٌ تَحْلِفُ بِآبَائِهَا فَقَالَ لاَ تَحْلِفُوا بِآبَائِكُمْ