Musannaf Ibn Abi Shayba 13:606

Abu Bakr narrated to us, he said: Ali ibn Mushir narrated to us, from Ubayd Allah, 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 there remains of his *kitābah* (contractual payment) even one dirham.