Musnad Ahmad 26:843

Hassan ibn Musa narrated to us, Shaiban narrated to us, from Abd al-Malik—meaning Ibn Umayr—from Abd al-Rahman ibn Abdullah—meaning Ibn Mas’ud—from his father, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Fighting a Muslim brother is disbelief, and reviling him is wickedness."