Musannaf Ibn Abi Shayba 13:1128

Abu Bakr narrated to us, he said: Hushaym narrated to us, from Yunus, from al-Hasan, regarding a slave owned by two men: He said: He used to dislike that one of them would enter into a *kitabah* (manumission contract) with him except with the permission of his partner. If he did so, the one who did not enter into the *kitabah* would share with him in everything he took from him. When the one who entered into the *kitabah* with him had completed what he had contracted him for, he would become free, and he would strive (work) for half of his value for the one who did not enter into the *kitabah* with him, and the *wala’* (patronage rights) would be between them.