Bulugh al-Maram, Book 16, Hadith 1487 (In-Book) ⓘ
Ibn Mas'ud (RAA) narrated that the Messenger of Allah said: insulting a Muslim is disobedience to Allah, and fighting with him is Kufr (disbelief).” Agreed upon.
وَعَنْ اِبْنِ مَسْعُودٍ - رضى الله عنه - قَالَ: قَالَ رَسُولُ اَللَّهِ - صلى الله عليه وسلم -{ سِبَابُ اَلْمُسْلِمِ فُسُوقٌ, وَقِتَالُهُ كُفْرٌ } مُتَّفَقٌ عَلَيْهِ. (1949). 4 - صحيح. رواه البخاري (6044) ومسلم (64).
also referenced under: Book 16, Hadith 1530 (English)