Mishkat al-Masabih 14 Emancipation كتاب العتق 18

وَعَنْ عَمْرِو بْنِ شُعَيْبٍ عَنْ أَبِيهِ عَنْ جَدِّهِ عَنِ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ: «الْمُكَاتَبُ عَبْدٌ مَا بَقِيَ عَلَيْهِ مِنْ مُكَاتبَته دِرْهَم» . رَوَاهُ أَبُو دَاوُد

‘Amr b. Shu’aib, on his father’s authority, told that this grandfather reported the Prophet as saying, “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.” Abu Dawud transmitted it.