Musnad Ahmad 26:628

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Mansur and Zubayd, from Abu Wa'il, from Abdullah, from the Prophet (peace and blessings of Allah be upon him) that he said: "Insulting a believer is wickedness (fisq), and fighting him is disbelief (kufr)." He said in the hadith of Zubayd: "I heard Abu Wa'il."