Musannaf Ibn Abi Shayba 37:323

'Abd al-Rahman ibn Muhammad al-Muharibi narrated to us, from Layth, from 'Awn ibn Abi Juhayfah, from 'Abd al-Rahman, from Ibn 'Umar, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Is one of you unable, when a man comes to him to kill him—meaning from such-and-such people—to say like this?" And he placed one of his hands upon the other. "Then he would be like the best of the two sons of Adam, and he would be in Paradise, while his killer would be in the Fire."