Ibn Mas'ud (RAA) narrated that the Messenger of Allah (ﷺ) said:
insulting a Muslim is disobedience to Allah, and fighting with him is Kufr (disbelief)." Agreed upon. (ID)
English reference (ID): Book 16, Hadith 1530 (ID)
Arabic reference (ID): Book 16, Hadith 1487 (ID)