Musnad Ahmad 25:1630

Yazid narrated to us, Hammad ibn Salamah informed us, from Ayyub, from Ikrimah, from Ibn Abbas, from the Prophet (peace and blessings of Allah be upon him), who said: "The *mukātab* (a slave under a contract of manumission) shall have his blood money paid according to the portion he has fulfilled as the blood money of a free man, and for what remains, as the blood money of a slave."