It is narrated from Sayyiduna Sa'd bin Abi Waqqas (may Allah be pleased with him) that he said: The Messenger of Allah (peace and blessings be upon him) said: "Killing a Muslim is disbelief, and reviling him is wickedness (fusuq), and it is not lawful for a Muslim to cut off his brother for more than three days."
Hadith Referenceالمعجم الكبير للطبراني / العشرة / 322