Musannaf Ibn Abi Shayba 18:1137

Abu Bakr narrated to us, he said: Isma'il ibn Ibrahim narrated to us, from Ayyub, from 'Ikrimah, who said: 'Ali said: "The *mukātab* (a slave under a contract of manumission) pays [the ransom/blood money] in proportion to what he has paid [of his contract]."