The Messenger of Allah ﷺ as saying: Your oath should be about something regarding which your companion will believe you. Musaddad said: 'Abd Allah b. Abi Salih narrated to me. Abu Dawud said: Both of them refer to the same person: 'Abbad b. Abu Salih and 'Abd Allah b. Abi Salih.  

قَالَ رَسُولُ اللَّهِ ﷺ يَمِينُكَ عَلَى مَا يُصَدِّقُكَ عَلَيْهَا صَاحِبُكَ قَالَ مُسَدَّدٌ قَالَ أَخْبَرَنِي عَبْدُ اللَّهِ بْنُ أَبِي صَالِحٍ  

We went out intending (to visit) the Messenger of Allah ﷺ and Wa'il ibn Hujr was with us. His enemy caught him. The people desisted from swearing an oath, but I took an oath that he was my brother. So he left him. We then came to the Messenger of Allah ﷺ, and I informed him that the people desisted from taking the oath, but I swore that he was my brother. He said: You spoke the truth: A Muslim is a brother of a Muslim.  

خَرَجْنَا نُرِيدُ رَسُولَ اللَّهِ وَمَعَنَا وَائِلُ بْنُ حُجْرٍ فَأَخَذَهُ عَدُوٌّ لَهُ فَتَحَرَّجَ الْقَوْمُ أَنْ يَحْلِفُوا وَحَلَفْتُ أَنَّهُ أَخِي فَخَلَّى سَبِيلَهُ فَأَتَيْنَا رَسُولَ اللَّهِ ﷺ فَأَخْبَرْتُهُ أَنَّ الْقَوْمَ تَحَرَّجُوا أَنْ يَحْلِفُوا وَحَلَفْتُ أَنَّهُ أَخِي قَالَ صَدَقْتَ الْمُسْلِمُ أَخُو الْمُسْلِمِ