Al-Adab al-Mufrad

It is narrated from Sayyiduna Abu Ayyub Ansari (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "It is not lawful for a Muslim to sever relations with his brother for more than three days, such that when they meet, one turns his face this way and the other turns his face that way. And the better of the two is the one who initiates the greeting of peace (salam)."

Sayyiduna Hisham bin ‘Amir (may Allah be pleased with him) reports that I heard the Messenger of Allah (peace and blessings be upon him) say: “It is not lawful for a Muslim to sever relations with another Muslim for more than three nights. Indeed, as long as they remain cut off from each other for more than three nights, both of them are away from the truth. Whoever among them returns to the truth first, that is, restores the relationship, his initiative will be an expiation for his previous severance. If both of them die in the state of severed relations, neither of them will enter Paradise.”