Musnad Ahmad 830:120

'Abd al-Samad narrated to us, Sa'id Abu 'Uthman narrated to us, in the square of al-Ahnaf, Muslim ibn Abi Bakrah narrated to us, from his father, that he heard the Prophet, may Allah bless him and grant him peace, saying: "If two Muslims fight each other, then the killer and the killed are in the Fire."