Sahih al-Bukhari 2679
Sahih al-Bukhari · Book 52: Witnesses · How to swear?
Narrated `Abdullah:
حَدَّثَنَا مُوسَى بْنُ إِسْمَاعِيلَ ، حَدَّثَنَا جُوَيْرِيَةُ ، قَالَ ذَكَرَ نَافِعٌ عَنْ عَبْدِ اللَّهِ ـ رضى الله عنه ـ أَنَّ النَّبِيَّ صلى الله عليه وسلم قَالَ " مَنْ كَانَ حَالِفًا فَلْيَحْلِفْ بِاللَّهِ أَوْ لِيَصْمُتْ " .
The Prophet (ﷺ) said, "Whoever has to take an oath should swear by Allah or remain silent." (i.e. He should not swear by other than Allah.)