Musnad Ahmad 32:1237

Rawh narrated to us, Ibn Jurayj and Zakariya ibn Ishaq narrated to us, from Ibn Shihab, Anas ibn Malik informed me, he said: The Messenger of Allah, may Allah bless him and grant him peace, said: "Do not cut ties with one another, do not turn your backs on one another, and do not hate one another. It is not lawful for a Muslim to forsake his brother for more than three nights."