The Prophet ﷺ said, "Reviling a Muslim is wickedness, 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 in the Book of Faith (64), both from the hadith of Shubah, from Zubayd, from Abu Waail, from Abdullah bin Masud, and he related the hadith. Zubayd said, I asked Abu Waail, "Did you hear it yourself from Abdullah, narrating it from the Messenger of Allah ﷺ?" He said, "Yes."Abu Dawud Tayalisi narrated it (204) from Shubah with the same wording, and he added at the end, "And it is not lawful for a Muslim to shun his brother for more than three days."