Musnad Abu Ya'la al-Mawsili 118:14

Abu Khaythama narrated to us, Ibn 'Uyayna narrated to us, from al-Zuhri, who heard it from Anas, from the Prophet, may Allah bless him and grant him peace, who said: "Do not hate one another, do not envy one another, do not turn your backs on one another, and do not sever ties with 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]."