Musannaf Ibn Abi Shayba 13:1401

Abu Bakr narrated to us, he said: Abbad narrated to us, from Hajjaj, from Muhammad ibn Qays ibn al-Ahnaf, from his father, from Ibn Mas’ud, regarding a man who sells the service of his *mudabbarah* (a slave woman promised freedom upon her master's death). He said: Whatever her master takes is his, and whatever remains, there is nothing [for the master].