Musannaf Ibn Abi Shayba 13:605

Abu Bakr narrated to us, he said: Isma'il ibn 'Ulayyah narrated to us, from Ayyub, from Nafi', from Ibn 'Umar, who said: The *mukātab* (a slave who has entered into a contract of manumission) is a slave as long as a dirham remains owed by him.