Musnad Ahmad 32:1410

Abu al-Yaman narrated to us, Shu'ayb informed us, from al-Zuhri, who said: Anas ibn Malik informed me that the Prophet, peace and blessings of Allah be upon him, said: "Do not hate one another, do not envy one another, and do not turn your backs on one another. Be, O servants of Allah, brothers. It is not lawful for a Muslim to forsake his brother for more than three nights, such that they meet and this one turns away and that one turns away, and the better of the two is the one who initiates the greeting."