مُوَطَّأُ مَالِكٍ
Muwatta Malik 1492
The Mukatab
حَدَّثَنِي مَالِكٌ، عَنْ نَافِعٍ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ، كَانَ يَقُولُ الْمُكَاتَبُ عَبْدٌ مَا بَقِيَ عَلَيْهِ مِنْ كِتَابَتِهِ شَىْءٌ .
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."
Arabic/English book reference: Book 39, Hadith 1