Musannaf Ibn Abi Shayba 13:1125

Abu Bakr narrated to us, he said: Sahl ibn Yusuf narrated to us, from Shu'bah, who said: I asked al-Hakam and Hammad about a slave who was owned by two men, and one of them entered into a *kitabah* (contract of manumission) for his share. Hammad disliked it, while al-Hakam saw no harm in it.