Musannaf Ibn Abi Shayba 13:580

Abu Bakr narrated to us, he said: Al-Fadl ibn Dukayn narrated to us, from Ibn Abi Ghunyah, from Al-Hakam, regarding a man who enters into a *kitabah* (contract of manumission) with a household collectively, then some of them die. He said: It is lifted from him in proportion to the share.