Musannaf Ibn Abi Shayba 13:1882

Abu Bakr narrated to us, he said: Ibn Mahdi narrated to us, he said: Hammad ibn Salamah narrated to us, from Abd al-Karim, from Amr ibn Shu’ayb and Ata, that Umar ibn al-Khattab said regarding the Jew, the Christian, and the slave: Their testimony is permissible.