Sahih Adab al-Mufrad

Narrated from Sayyiduna Anas bin Malik (may Allah be pleased with him) that the Messenger of Allah (peace and blessings of Allah be upon him) said: “Do not bear malice against one another, do not envy one another, do not boycott one another, and O servants of Allah! Be brothers to one another. It is not lawful for any Muslim to boycott his Muslim brother for more than three days. Do not speak ill behind each other's backs, and O servants of Allah, live as brothers among yourselves. It is not lawful for any Muslim to sever relations with his brother for more than three nights.”