Sayyiduna Anas bin Malik (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "Do not envy one another, do not sever ties with one another, and do not bear enmity towards one another; and, O servants of Allah, be brothers. It is not permissible for a Muslim to stop speaking to his Muslim brother for more than three days."
(b) Ibn Umar (may Allah be pleased with them both) narrates from the Prophet (peace and blessings be upon him): "It is not permissible for a Muslim to stop speaking to his Muslim brother for more than three days."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب القسم والنشوز / 14773