Musannaf Ibn Abi Shayba 18:475

Abu Bakr narrated to us, he said: Abbad narrated to us, from Umar ibn Amir, from Hammad, from Ibrahim, regarding the slave who kills a free man intentionally: He said: They do not have the right to use him for service. They only have his blood; if they wish, they may kill him, and if they wish, they may pardon him.