Abu Dawud Book 24, Hadith Number 3915.

Chapter : A slave who has entered into an agreement to purchase his freedom mukatab and made a part payment but finally he is unable to complete the amount or dies.

Narated By Abdullah ibn Amr ibn al-‘As : The Prophet (PBUH) said: A slave who has entered into an agreement to purchase his freedom is a slave as long as a dirham of the agreed price remains to be paid.