The only admissible oath for (establishing) rights (through the courts) is swearing by Allah (Exalted is He), whether the person taking the oath is a Muslim or an unbeliever.
اْليَمِيْنُ الْمَشْرُوْعَةُ فِيْ اْلحُقُوْقِ، هِيَ اْليَمِيْنُ بِاللهِ تَعَالى، سَوَاءٌ كَانَ اْلحَالِفُ مُسْلِمًا، أَوْ كَافِرًا.