The Prophet ﷺ said, "Cursing a Muslim is grave disobedience, and fighting him is disbelief."
حدثنا أبو بكر بن أبي شيبة حدثنا محمد بن الحسن الأسدي حدثنا أبو هلال عن ابن سيرين
عن النبي ﷺ قال سباب المسلم فسوق وقتاله كفر
[Sahih]. Aqeeli reported it in Duafa 4/50, from Ibn Abu Shayba through this chain, and Busiri graded it hasan. Ibn Awn also reported it from Ibn Sirin through this chain in Tarikh Baghdad by Khatib, 3/397, and in Hilyat al-Awliya, 8/359, in the narration of Munakhkhal bin Hakeem al-Qushayri. The preceding hadith serves as a corroborating witness for it.