The Messenger of Allah ﷺ said, "A Muslim's fighting his brother is disbelief, and reviling him is wickedness."
حدثنا محمد بن عبد الله بن بزيع حدثنا عبد الحكيم بن منصور الواسطي عن عبد الملك بن عمير
قال رسول الله ﷺ قتال المسلم أخاه كفر وسبابه فسوق
[Sahih]. Nasai also narrated it: 7/122, no. 113 (Prohibition of Blood, Chapter on a Muslim fighting) from the hadith of Abdul-Malik bin Umayr through this route. The hadith has many corroborating narrations, including the one that follows.Other narrations on this topic come from Saad bin Abu Waqqas [Ibn Majah, no. 3941] and Abdullah bin Mughafal [Tabarani in Awsat: 1/413, no. 738].