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