Musnad Ahmad 32:133

Sufyan narrated to us, from al-Zuhri, who heard it from Anas, from the Prophet, peace and blessings of Allah be upon him, who said: "Do not cut ties with one another, do not hate one another, do not turn your backs on one another, and do not envy one another. And be, servants of Allah, brothers. And it is not lawful for a Muslim to forsake his brother for more than three [days]."