The Messenger of Allah ﷺ found 'Umar al-Khattab in a caravan while he was swearing by his father. So he said: Allah forbids you to swear by forefathers. If anyone swears, he must swear by Allah or keep silence.  

أَنَّ رَسُولَ اللَّهِ ﷺ أَدْرَكَهُ وَهُوَ فِي رَكْبٍ وَهُوَ يَحْلِفُ بِأَبِيهِ فَقَالَ إِنَّ اللَّهَ يَنْهَاكُمْ أَنْ تَحْلِفُوا بِآبَائِكُمْ فَمَنْ كَانَ حَالِفًا فَلْيَحْلِفْ بِاللَّهِ أَوْ لِيَسْكُتْ  

The Prophet ﷺ said: He who swears by Amanah (faithfulness) is not one of us.  

قَالَ رَسُولُ اللَّهِ ﷺ مَنْ حَلَفَ بِالأَمَانَةِ فَلَيْسَ مِنَّا  

The Prophet ﷺ said: If anyone takes an oath and says: I am free from Islam; now if he is a liar (in his oath), he will not return to Islam with soundness.  

قَالَ رَسُولُ اللَّهِ ﷺ مَنْ حَلَفَ فَقَالَ إِنِّي بَرِيءٌ مِنَ الإِسْلاَمِ فَإِنْ كَانَ كَاذِبًا فَهُوَ كَمَا قَالَ وَإِنْ كَانَ صَادِقًا فَلَنْ يَرْجِعَ إِلَى الإِسْلاَمِ سَالِمًا