Musnad Ahmad 27:1258

Yahya ibn Hammad narrated to us, Abu 'Awanah narrated to us, from Raqabah, from 'Awn ibn Abi Juhayfah, from 'Abd al-Rahman ibn Samurah, who said: I was walking with 'Abdullah ibn 'Umar, and we came upon a head set up on a piece of wood. He said: "Wretched is the killer of this one." I said: "Do you say this, O Abu 'Abd al-Rahman?" He pulled his hand away from my hand and said: "Abu 'Abd al-Rahman heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'If a man from my nation walks toward a man to kill him, let him say such and such; the killed is in Paradise, and the killer is in the Fire.'"