Abu Dawud Book 30, Hadith Number 4247.
Narated By Abdullah ibn Umar : Abdur Rahman 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 (PBUH) 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.
Abu Dawud said: Al-Thawri has transmitted it from ‘Awn from ‘Abd al-Rahman b. Sumair or Sumairah; and Laith b. Abi Sulaim transmitted from ‘Awn from ‘Abd al-Rahman b. Sumairah.
Abu Dawud said: Al-Hasan b. ‘Ali said to me: Abu al-Walid transmitted this tradition to us from Abu ‘Awanah, and said: It (the name Ibn Samurah) is in my notebook Ibn Sabrah. The people also transmitted it as Samurah and Sumuirah. These are wordings of Abu al-Walid.