مِشْكَاةُ الْمَصَابِيحِ
Mishkat al-Masabih 3415
Oaths and Vows
Section 1
وَعَنْهُ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «يَمِينُكَ عَلَى مَا يُصَدِّقُكَ عَلَيْهِ صَاحبك» . رَوَاهُ مُسلم
He reported God’s Messenger as saying, "Your oath should be about something regarding which your companion will believe you.” Muslim transmitted it.
Mishkat al-Masabih, 3415صَحِيحٌ (الألباني)