Musannaf Ibn Abi Shayba 18:602

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Ibn Abi Dhi’b, from the son of Muhammad ibn Ibrahim al-Taymi, from his father, from al-Saluli, from Mu’adh ibn Jabal, from Abu Ubaydah ibn al-Jarrah, who said: The crime of the *mudabbar* (a slave promised freedom upon his master's death) is upon his master.