Musannaf Ibn Abi Shayba 13:616

Abu Bakr narrated to us, he said: Hafs bin Ghiyath narrated to us, from al-A’mash, from Ibrahim, and from Ash’ath, from al-Sha’bi; they both said: Abdullah said: "When the *mukātab* (a slave under a contract of manumission) has paid a third of his *kitābah* (contractual amount), he is a creditor."