Mishkat al-Masabih #3281
Oaths and Vows
The Arabic includes the chain of narration (sanad) and the collector's commentary; the English translation shows the Prophet's words only.
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.
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About Mishkat al-Masabih
This hadith is #3281 from Mishkat al-Masabih, in the Book of Oaths and Vows.