Musnad Abu Ya'la al-Mawsili 118:15

Muhammad ibn 'Abbad al-Makki narrated to us, Sufyan narrated to us, he said: I memorized these four from al-Zuhri, that he heard Anas ibn Malik saying: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Do not cut ties with one another, do not hate one another, do not envy one another, and do not turn your backs on 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]."