The Prophet ﷺ said, "Reviling a Muslim is grave disobedience, and fighting him is disbelief."
عن النبي ﷺ قال: سباب المسلم فسوق، وقتاله كفر.
Bukhari narrated it in the Book of Faith (48), and Muslim narrated it in the Book of Faith (64), both from the hadith of Shubah, from Zubayd, from Abu Waail, from Abdullah bin Masud, and he related it.