Musnad Ahmad 30:1801

Qutaybah narrated to us, 'Abd al-'Aziz ibn Muhammad narrated to us, from al-'Ala' ibn 'Abd al-Rahman, from his father, from Abu Hurayrah, that the Messenger of Allah (may Allah bless him and grant him peace) said: "The disbeliever and the one who killed him shall never be gathered together in the Fire."