Mishkat al-Masabih 15 Oaths and Vows كتاب الإيمان والنذور 10

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.