Musnad Abu Ya'la al-Mawsili 108:165

Abu Bakr narrated to us, Sharik narrated to us, from Salamah ibn Kuhayl, from ‘Ata’ and Abu al-Zubayr, from Jabir: That the Prophet (peace and blessings of Allah be upon him) sold a *mudabbar* (a slave promised freedom upon the master's death).