The Prophet ﷺ said, "Fighting a Muslim is disbelief, and reviling him is wickedness."
أن النبي ﷺ قال: قتال المسلم كفر، وسبابه فسق.
Ahmad (1537), Bukhari in Adab al-Mufrad (429), and Ibn Nasr in Tazim Qadr Salah (1099) all narrated it from the hadith of Zakariya bin Abu Zaida, from Abu Ishaq, from Muhammad bin Saad bin Malik, from his father, who related it. Its chain of transmission is sahih.Ibn Majah (3941) narrated it from the hadith of Sharik, from Abu Ishaq, with his chain of transmission, similarly. Sharik is Ibn Abdullah Nakhai, whose memory was poor, but he is acceptable as a corroborating narrator, as is the case here.