Musnad Ahmad 30:2064

Mu'awiyah narrated to us, he said: Abu Ishaq 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: "They shall never be gathered in the Fire together in a gathering that harms one of them." They said: "Who, O Messenger of Allah?" He said: "A believer whom a disbeliever kills, then he (the disbeliever) acts righteously thereafter."