Musnad Ahmad 26:407

Hisham ibn ‘Abd al-Malik narrated to us, Abu ‘Awanah narrated to us, from ‘Abd al-Malik, 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 a Muslim is disbelief, and reviling him is wickedness."