Ata' bin Yazid (may Allah have mercy on him) said: I heard Sayyiduna Ubayy bin Ka'b (may Allah be pleased with him) say: The Messenger of Allah (peace and blessings be upon him) said: "It is not lawful for a Muslim to cut off his brother for more than three days; then if they meet, this one turns away and that one turns away, and the better of the two is the one who greets first with salam."
Hadith Referenceالمعجم الكبير للطبراني / باب الخاء / 3945