The Prophet ﷺ said, "Fighting a Muslim is disbelief, and reviling him is wickedness."
حدثنا علي بن بحر حدثنا عيسى بن يونس عن زكريا عن أبي إسحاق
أن النبي ﷺ قال قتال المسلم كفر وسبابه فسق
Its chain of transmission is sahih, and it is an abridgment of 1519. Reference to this chain has already been made there.