Musnad Ahmad 8:81

'Abd al-Razzaq narrated to us, Ma'mar informed us, from Abu Ishaq, from 'Umar ibn Sa'd, Sa'd ibn Abi Waqqas narrated to us, he said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "Fighting a Muslim is disbelief, and reviling him is wickedness. And it is not lawful for a Muslim to shun his brother for more than three days."