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. (FR)
English reference (FR): Book 16, Hadith 1530 (FR)
Arabic reference (FR): Book 16, Hadith 1487 (FR)