Musnad Ahmad 25:1564

Ya'la narrated to us, Hajjaj al-Sawwaf narrated to us, from Yahya, from Ikrimah, from Ibn Abbas, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, decreed regarding the *mukātab* (a slave who has a contract of manumission) who is killed: that for what he has paid of his *kitābah* (contract), the blood money of a free man is paid, and for what remains, the blood money of a slave is paid."