The Messenger of Allah ﷺ said, "Reviling a Muslim is wickedness, and fighting him is disbelief."
أخبرنا محمود بن غيلان قال حدثنا وهب بن جرير قال حدثنا أبي قال سمعت عبد الملك بن عمير يحدثه
أن رسول الله ﷺ قال سباب المسلم فسوق وقتاله كفر
[Sahih, marfu] Reported by Tirmidhi, Chapter on Trials, section on the statement that reviling a Muslim is transgression, no. 2634, from the hadith of Abdul-Malik bin Umayr with this wording, and he said, hasan sahih. It is also found in Al-Kubra, no. 3571, and the hadith has many corroborating witnesses.