The Messenger of Allah ﷺ said, "Reviling a Muslim is grave disobedience, and fighting him is disbelief."Zubayd said, I asked Abu Waail, "Did you yourself hear this from Abdullah?"He replied, "Yes."
حدثنا محمود بن غيلان حدثنا وكيع حدثنا سفيان عن زبيد بن الحارث عن أبي وائل
قال رسول الله ﷺ سباب المسلم فسوق وقتاله كفر قال زبيد قلت لأبي وائل أأنت سمعته من عبد الله قال نعم
Agreed upon [by Bukhari and Muslim]. Muslim recorded it in the Book of Faith, chapter on the statement of the Prophet ﷺ, "Reviling a Muslim is immorality, and fighting him is disbelief," hadith no. 64, from the hadith of Sufyan Thawri, and Bukhari, hadith no. 48, from the hadith of Zubayd, transmitting it in the same way.