Musannaf Ibn Abi Shayba 13:2097

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Hammad bin Zayd narrated to us, from Ibn Abi Dhi’b, from Qariẓ bin Shaybah, from Sa’id bin al-Musayyib, he said: "There is no harm in the service of the *mudabbar* (a slave promised freedom upon his master's death)." And al-Zuhri used to say it.