“The oath is based upon what will make your companion believe you.” [Qutaibah (one of the narrators) said: “ What will make you believed by your companion”]  

قَالَ رَسُولُ اللَّهِ ﷺ الْيَمِينُ عَلَى مَا يُصَدِّقُكَ بِهِ صَاحِبُكَ وَقَالَ قُتَيْبَةُ عَلَى مَا صَدَّقَكَ عَلَيْهِ صَاحِبُكَ