Musannaf Ibn Abi Shayba 13:1123

Abu Bakr narrated to us, he said: Abdullah ibn al-Mubarak narrated to us, from Ya'qub ibn al-Qa'qa', from Matar, from al-Hasan, regarding a slave owned by three people, [where] one of them entered into a *kitabah* (manumission contract) with him. He said: Whatever is taken from him is taken, and it is divided among his partners, and the slave remains between them; his *kitabah* is not valid.