Musnad Ahmad 32:1110

'Abd al-A'la narrated to us, from Ma'mar, from al-Zuhri, from Anas ibn Malik, that the Messenger of Allah, peace and blessings of Allah be upon him, said: "Do not hate one another, do not cut off relations with one another, do not turn your backs on one another, and do not envy one another. And be, O servants of Allah, brothers. It is not lawful for a Muslim to forsake his brother for more than three days."