The Prophet ﷺ said, "Insulting a Muslim is wickedness, and fighting him is disbelief."
أن النبي ﷺ قال: سباب المسلم فسوق وقتاله كفر.
Bukhari narrated it in the Book of Faith (48) and Muslim in the Book of Faith (64), both from the hadith of Shubah, from Zubayd, from Abu Waail, from Abdullah bin Masud, who mentioned it.