Musnad Ahmad 26:794

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