Sahih al-Bukhari 6108
Sahih al-Bukhari · Book 78: Good Manners and Form (Al-Adab) · Whoever does not consider as Kafir the person who says that...
Narrated Ibn `Umar:
حَدَّثَنَا قُتَيْبَةُ ، حَدَّثَنَا لَيْثٌ ، عَنْ نَافِعٍ ، عَنِ ابْنِ عُمَرَ ـ رضى الله عنهما أَنَّهُ أَدْرَكَ عُمَرَ بْنَ الْخَطَّابِ فِي رَكْبٍ وَهْوَ يَحْلِفُ بِأَبِيهِ، فَنَادَاهُمْ رَسُولُ اللَّهِ صلى الله عليه وسلم " أَلاَ إِنَّ اللَّهَ يَنْهَاكُمْ أَنْ تَحْلِفُوا بِآبَائِكُمْ، فَمَنْ كَانَ حَالِفًا فَلْيَحْلِفْ بِاللَّهِ، وَإِلاَّ فَلْيَصْمُتْ " .
that he found `Umar bin Al-Khattab in a group of people and he was swearing by his father. So Allah's Messenger (ﷺ) called them, saying, "Verily! Allah forbids you to swear by your fathers. If one has to take an oath, he should swear by Allah or otherwise keep quiet."