The Messenger of Allah ﷺ said, "Insulting a Muslim is wickedness, and fighting him is disbelief."
حدثنا علي بن محمد حدثنا وكيع عن شريك عن أبي إسحق عن محمد بن سعد
قال رسول الله ﷺ سباب المسلم فسوق وقتاله كفر
[Sahih]Takhrij: Ahmad narrated it (1/178) and others, from the hadith of Abu Ishaq, with this chain, and Busiri graded it sahih. Zakariya bin Zaidah and Israil also narrated it from Abu Ishaq with this chain, and Maamar narrated it from Abu Ishaq from Umar bin Saad from Saad with this chain (Nasai, 7/121, h. 4115). This hadith has many corroborating narrations, see h. 3939.