The Prophet ﷺ said, "Reviling a Muslim is grave disobedience, and fighting him is disbelief."
عن النبي ﷺ، قال: سباب المسلم فسوق وقتاله كفر.
Ibn Majah (3940) narrated it from Abu Bakr bin Abu Shayba, who said, Muhammad bin Hasan Asadi told us, who said, Abu Hilal told us, from Ibn Sirin, from Abu Hurairah, and he related it.Its chain of transmission is hasan because of Muhammad bin Hasan Asadi and his teacher Abu Hilal, who is Muhammad bin Sulaym Rasibi, for there is disagreement about both of them, except that their hadiths are hasan when they do not contradict others and do not bring forth anything that would be rejected from them.