Riyad as-Salihin
Riyad as-Salihin 1559
Prohibition of Reviling a Muslim without any cause
Arabic hadith text
وعن ابن مسعود رضي الله عنه قال: قال رسول الله صلى الله عليه وسلم : "سباب المسلم فسوق، وقتاله كفر" ((متفق عليه)).
Translation
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].