Musnad Ahmad 28:445

Yahya ibn Zakariya ibn Abi Za'idah narrated to us, Hajjaj informed us, from 'Amr ibn Shu'ayb, from his father, from his grandfather, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Any slave who enters into a contract of manumission (kitabah) for one hundred uqiyahs, and then pays it off except for ten uqiyahs, and then becomes unable (to pay the remainder), he is a slave."