Musannaf Ibn Abi Shayba 13:1320

Abu Bakr narrated to us, he said: Hushaym narrated to us, from Yunus, from al-Hasan, who said: If they wish to sell him, they may sell him, and if they wish, they may make him work to earn his freedom (istasa'uhu). He said: Shurayh used to say that.