Musnad Ahmad 1018:5

Ya'la ibn 'Ubayd narrated to us, Muhammad narrated to us, from Yazid ibn Abi Habib, from Marthad ibn 'Abdullah, from a man among the companions of the Prophet, may Allah bless him and grant him peace, who said: The Messenger of Allah, may Allah bless him and grant him peace, was asked about the killer and the one who orders [the killing]. He said: "The Fire is divided into seventy parts; the one who orders has sixty-nine, and the killer has one part, and that is sufficient for him."