The Messenger of Allah ﷺ said, "A Muslim's fighting his brother is disbelief, and reviling him is wickedness."
حدثنا حسن بن موسى حدثنا شيبان عن عبد الملك يعني ابن عمير
قال رسول الله ﷺ قتال المسلم أخاه كفر وسبابه فسوق
Its isnad is sahih, and it is a repetition of 4345.