Musannaf Ibn Abi Shayba 13:2922

Abu Bakr narrated to us, he said: Hafs narrated to us, from Ash'ath, from al-Hakam and Hammad, from Ibrahim or al-Sha'bi or al-Hasan, he said: The testimony of the people of a religion is not permissible except against the people of their own religion; the Jew against the Jew, and the Christian against the Christian.