Ibn Mas'ud (May Allah be pleased with him) reported:
The Messenger of Allah (ﷺ) said, "Reviling a Muslim is Fusuq (disobedience of Allah) and killing him is (tantamount to) disbelief."[Al-Bukhari and Muslim].
App reference: Book 18, Hadith 49
Arabic/English book reference: Book 18, Hadith 1559