Musnad Ahmad 25:107

Isma'il narrated to us, Hisham informed us, from Yahya ibn Abi Kathir, from 'Ikrimah, from Ibn 'Abbas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said regarding the *mukātab* (a slave who has a contract of manumission): "He is emancipated from [his status as a slave] in proportion to what he has paid of the blood money of a free man, and in proportion to what remains of his servitude [he is treated] as the blood money of a slave."