Musnad Ahmad 26:353

'Affan narrated to us, Shu'bah narrated to us, he said: Zubayd, and Mansur, and Sulayman informed me that they heard Abu Wa'il narrating from 'Abdullah, from the Prophet (peace and blessings of Allah be upon him), who said: "Insulting a Muslim is an act of wickedness (fisq), and fighting him is an act of disbelief (kufr)." Zubayd said: So I said to Abu Wa'il, twice: "Did you hear it from 'Abdullah, from the Prophet (peace and blessings of Allah be upon him)?" He said: "Yes."