Hadith 514910
Chapter 39: The Mukatab - كتاب المكاتب
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."
Reference Muwatta Malik 514910
USC-MSA web (English) reference : Book 39, Hadith 1