Musnad Abu Ya'la al-Mawsili 152:264

Abu Khaythama narrated to us, Yahya ibn Abi Bukayr narrated to us, Shayban narrated to us, from Abd al-Malik ibn Umayr, from Abd al-Rahman ibn Abd Allah ibn Mas'ud, from his father, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Fighting the believer is disbelief, and reviling him is wickedness."