Musannaf Ibn Abi Shayba 13:1821

Abu Bakr narrated to us, he said: Ma'n ibn 'Isa narrated to us, from Ibn Abi Dhi'b, from al-Zuhri, who said: I asked about the judgment of the Jews and the Christians when they bring their disputes to us. He said: Judge between them with your judgment for the Muslims; it is not permissible between them except what is permissible between the Muslims.