Musannaf Ibn Abi Shayba 18:1136

Abu Bakr narrated to us, he said: Ibn ‘Ulayyah narrated to us, from Hisham al-Dastawa’i, from Yahya ibn Abi Kathir, from ‘Ikrimah, from Ibn ‘Abbas, he said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The *mukātab* (a slave under a contract of manumission) shall pay, in proportion to what has been emancipated of him, the blood money of a free man, and in proportion to what remains of his servitude, the blood money of a slave."