Musannaf Ibn Abi Shayba 13:2918

Abu Bakr narrated to us, he said: Ibn Idris narrated to us, from Layth, from 'Ata', who said: The testimony of a Jew against a Christian is not permissible, nor [the testimony of] a Christian against a Magian, nor [the testimony of] any religious community against a community other than its own, except for the Muslims.