Riyad as-SalihinChapter 17Hadith 880
Full Sharh — Riyad as-Salihin 880
وعن ابن مسعود رضي الله عنه قال: قال رسول الله صلى الله عليه وسلم : "سباب المسلم فسوق، وقتاله كفر" ((متفق عليه)).
English 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.".
Source
Riyad as-Salihin(رياض الصالحين)
The Book of the Prohibited actions
Hadith Number
880