Musnad Ahmad 28:190

Ibn Numayr narrated to us, Hajjaj narrated to 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 awqiyah, and then pays it off except for ten awqiyah, he remains a slave."