Musnad Abu Ya'la al-Mawsili 155:212

Abu Bakr ibn Abi Shaybah narrated to us, Muhammad ibn al-Hasan al-Asadi narrated to us, Abu Hilal narrated to us, Muhammad ibn Sirin narrated to us, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Insulting a believer is wickedness (fisq), and fighting him is disbelief (kufr)."