Compiled by Imam Yahya ibn Sharaf al-Nawawi
وعن ابن مسعود رضي الله عنه قال: قال رسول الله صلى الله عليه وسلم : "سباب المسلم فسوق، وقتاله كفر" ((متفق عليه)).
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].