Musnad Abu Ya'la al-Mawsili 151:25

Abu Bakr narrated to us, Mu'tamir narrated to us from his father, Abu 'Amr al-Shaybani narrated to us from 'Abdullah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Insulting a Muslim is wickedness (fisq), and fighting him is disbelief (kufr)."