Musannaf Ibn Abi Shayba 18:469

Abu Bakr narrated to us, he said: Hatim ibn Isma'il narrated to us, from Ja'far, from his father, from Ali, he said: If a slave kills a free person, he is handed over to the guardians of the slain. If they wish, they may kill him, and if they wish, they may spare his life.