Musnad Ahmad 5:161

'Affan narrated to us, Wuhayb narrated to us, Ayyub narrated to us, from 'Ikrimah, from 'Ali ibn Abi Talib, from the Prophet, peace and blessings of Allah be upon him, who said: "The *mukātab* (a slave under a contract of manumission) is to be paid [the blood money] in proportion to what he has paid [of his ransom]."