Trials and Fierce Battles (Kitab Al-Fitan Wa Al-Malahim)
Dawud :: Book 35 : Hadith 4247
Narrated Abdullah ibn Umar:
AbdurRahman ibn Samurah said: I was holding the hand of Ibn Umar on one of the ways of Medina. He suddenly came to a hanging head. He said: Unhappy is the one who killed him. When he proceeded, he said: I do not consider him but unfortunate. I heard the Apostle of Allah (peace_be_upon_him) say: If anyone goes to a man of my community in order to kill him, he should say in this way, the one who kills will go to Hell and the one who is killed will go to Paradise.