Musannaf Ibn Abi Shayba 18:982

Abu Bakr narrated to us, he said: Harami ibn 'Umarah narrated to us, from Shu'bah, he said: I asked al-Hakam and Hammad about two men who testified against a man, so his hand was cut off. Then they looked, and behold, one of the two witnesses was a slave. They both said: The Imam shall provide compensation.