It is narrated from Sayyiduna Hisham bin Amir al-Ansari (may Allah be pleased with him); he said: I heard the Messenger of Allah (peace and blessings be upon him) say: "It is not lawful for a Muslim to cut off relations with a Muslim for more than three days. As long as the two of them remain cut off, they are turned away from the truth. Whichever of them is first to reconcile, his being first will be an expiation for him. If the two of them die in their state of severed relations, they will never both enter Paradise together. And if one greets the other with salam and the other does not accept his reconciliation and his greeting, an angel will reply to the one who greeted, and Satan will reply to the other."
Hadith Referenceالمعجم الكبير للطبراني / باب الهاء / 18675