"The Messenger of Allah said: 'Whoever swears, let him not swear by anything other than Allah.'" The Quraish used to swear by their forefathers, and so he said: "Do not swear by your forefathers."  

قَالَ رَسُولُ اللَّهِ ﷺ مَنْ كَانَ حَالِفًا فَلاَ يَحْلِفْ إِلاَّ بِاللَّهِ وَكَانَتْ قُرَيْشٌ تَحْلِفُ بِآبَائِهَا فَقَالَ لاَ 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.'"'"  

قَالَ رَسُولُ اللَّهِ ﷺ إِنَّ اللَّهَ يَنْهَاكُمْ أَنْ تَحْلِفُوا بِآبَائِكُمْ