Musannaf Ibn Abi Shayba 13:326

Abu Bakr narrated to us, he said: Hafs bin Ghiyath narrated to us, from Ash'ath, from al-Sha'bi, he said: Shurayh said: "The testimony of slaves is not permitted." Ali said: "We used to permit it." He said: So Shurayh used to permit it after that, except for their master.