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].

وعن ابن مسعود رضي الله عنه قال: قال رسول الله ﷺ : "سباب المسلم فسوق، وقتاله كفر" ((متفق عليه)).