Muwatta Malik · Hadith 1489
حدثني مالك، عن نافع، ان عبد الله بن عمر، كان يقول المكاتب عبد ما بقي عليه من كتابته شىء
Malik related to me from Nafi that Abdullah ibn Umar said, "A mukatab is a slave as long as any of his kitaba remains to be paid
Mauquf Sahih · Salim al-Hilali