Ibn Abbas (may Allah be pleased with them both) narrates from the Prophet (peace and blessings be upon him): "The mukatab will pay the blood money (diyah) of a free man in proportion to the part of him that is free, and the blood money of a slave for the rest."
Al-Bukhari, when asked about this hadith, said: Some narrated this hadith from Ayyub, from Ikrimah, from Ali (may Allah be pleased with him). The Shaykh said: He meant the second hadith; as for the first, it is among the solitary narrations of Hammad.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21653