Your oath should be about something regarding which your companion will believe you. 'Amr said: By which your companion will believe you.
قَالَ رَسُولُ اللَّهِ ﷺ يَمِينُكَ عَلَى مَا يُصَدِّقُكَ عَلَيْهِ صَاحِبُكَ وَقَالَ عَمْرٌو يُصَدِّقُكَ بِهِ صَاحِبُكَ