The Messenger of Allah ﷺ said, "A Muslim's fighting his brother is disbelief, and reviling him is grave disobedience."
حدثنا هشام بن عبد الملك حدثنا أبو عوانة عن عبد الملك
قال رسول الله ﷺ قتال مسلم أخاه كفر وسبابه فسوق
Its isnad is sahih, and it has previously appeared through the route of Abu Waail from Ibn Masud at 3647 and 3903.