كتاب الأمور المنهي عنها
18
The Book of the Prohibited actions
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 صلى الله عليه وسلم ‏:‏ ‏ "‏سباب المسلم فسوق، وقتاله كفر‏"‏ ‏(‏‏(‏متفق عليه‏)‏‏)‏‏.‏
Sunnah.com reference : Book 18, Hadith 49
Arabic/English book reference : Book 18, Hadith 1559