Musnad Ahmad 25:498

Muhammad ibn Abdullah narrated to us, Hisham ibn Abi Abdullah narrated to us, Yahya ibn Abi Kathir narrated to us, from Ikrimah, from Ibn Abbas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The *mukātab* (a slave under a contract of manumission) is to be compensated for the portion he has paid off with the blood money of a free man, and for the portion he remains a slave with the blood money of a slave."