Musnad Ahmad 26:100

Yahya narrated to us, from Shu'bah, Zubayd narrated to me, from Abu Wa'il, from Abdullah, from the Prophet (peace and blessings of Allah be upon him), who said: "Insulting a Muslim is wickedness (fisq), and fighting him is disbelief (kufr)." He said: I said to Abu Wa'il, "Did you hear this from Abdullah?" He said, "Yes."