Sahih Adab al-Mufrad

It is narrated from Hisham bin 'Amir Ansari, who was a cousin of Anas bin Malik and whose father was martyred on the day of Uhud, that he said he heard the Messenger of Allah (peace and blessings be upon him) say: "It is not permissible for a Muslim to cut off relations with another Muslim for more than three days. As long as both of them persist in this estrangement, they remain away from the truth. And whoever reconciles first, his initiative will be an expiation for his sin. If they die in this state of estrangement, they will never enter Paradise together. And if one of them greets the other and the other does not accept his greeting, then an angel responds to the one who greeted, and Satan responds to the one who refused. Whoever ends this situation first, his action will be an expiation for the previous mistake. And if both die in this state, neither of them will ever enter Paradise. And if one greets and the other refuses to accept the greeting, then an angel responds to the one who greeted, and Satan responds to the one who refused."