Musannaf Ibn Abi Shayba 13:702

Abu Bakr narrated to us, he said: Hafs and Abu Khalid narrated to us, from Hajjaj, from al-Hasan ibn al-Hakam, from Zayd ibn Thabit; and Hajjaj [narrated] from al-Hakam, from Shurayh; they both said: The *mudabbar* (a slave promised freedom upon the master's death) is not to be sold.