The Prophet ﷺ said, "Reviling a Muslim is wickedness, and fighting him is disbelief."
عن النبي ﷺ قال: سباب المسلم فسوق وقتاله كفر.
Marwazi narrated it in Tazim Qadr al-Salah (1104), from Ali bin Hasan bin Abu Isa, who said, Muqri narrated to us, who said, Ibn Lahia narrated to us, who said, Yazid bin Abu Habib narrated to me, from Sinan bin Saad Kindi, from Anas, and he mentioned it.Its chain is hasan, because of Sinan bin Saad, also said to be Saad bin Sinan. There is disagreement about him, but his hadith is hasan when his errors are not apparent, since he erred frequently, though his hadith has a basis. As for Ibn Lahia, Abdullah bin Yazid Muqri narrated from him, and the narration of the Abadila from him is more reliable than that of others.