Musnad Abu Ya'la al-Mawsili 152:208

Abu Khaythama narrated to us, he said: Abd al-Rahman ibn Mahdi narrated to us, from Sufyan, from Zubayd, from Abu Wa'il, from Abdullah ibn Mas'ud, from the Prophet, peace and blessings of Allah be upon him, who said: "Insulting the Muslim—or the believer—is wickedness (fisq), and fighting him is disbelief (kufr)." I said to Abu Wa'il: "Did you hear Ibn Mas'ud narrating it from the Prophet, peace and blessings of Allah be upon him?" He said: "Yes."