Musnad Ahmad 30:1516

Hassan narrated to us, Hammad ibn Salamah narrated to us, from Suhayl ibn Abi Salih, from his father, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "A believer who killed a disbeliever and then acted righteously thereafter shall not be gathered together in the Fire in a way that harms him."