The oath must be taken by the defendant

that the Prophet ﷺ said, "If the people were given according to their claims, they would claim the lives of persons and their properties, but the oath must be taken by the defendant."  

أَنَّ النَّبِيَّ ﷺ قَالَ: «لَوْ يُعْطَى النَّاسُ بِدَعْوَاهُمْ لاَدَّعَى نَاسٌ دِمَاءَ رِجَالٍ وَأَمْوَالَهُمْ وَلَكِنَّ الْيَمِينَ عَلَى الْمُدَّعَى عَلَيْهِ۔»  

Ibn 'Abbas reported that Messenger of Allah ﷺ pronounced judgment on the basis of oath by the defendant.  

أَنَّ رَسُولَ اللَّهِ ﷺ قَضَى بِالْيَمِينِ عَلَى الْمُدَّعَى عَلَيْهِ