Musnad Abu Ya'la al-Mawsili 130:21

Muhammad ibn Abi Bakr al-Muqaddami narrated to us, Mubarak narrated to us, from Abd al-Aziz ibn Suhayb, from Anas, who said: The Messenger of Allah (peace be upon him) said: "There will be people in my nation who will pass through the religion just as an arrow passes through the game. They are the worst of the slain under the shade of the sky. Blessed is he who killed them, blessed is he whom they killed, blessed is he whom they killed."