Musannaf Ibn Abi Shayba 13:1587

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Al-Rabi’ ibn Sabih narrated to us, from Yazid ibn Aban, from Sabih ibn Abdullah, that Abdullah ibn Abbas urged the people regarding his *mukātab* (slave with a contract of manumission). So they collected [funds] for him, and he paid off his *mukātabah* (contract), and a surplus remained. Abdullah placed it among the *mukātabīn* (those with contracts of manumission).