بُلُوغُ الْمَرَامِ
Bulugh al-Maram 1487
The Comprehensive Book
وَعَنْ اِبْنِ مَسْعُودٍ - رضى الله عنه - قَالَ: قَالَ رَسُولُ اَللَّهِ - صلى الله عليه وسلم -{ سِبَابُ اَلْمُسْلِمِ فُسُوقٌ, وَقِتَالُهُ كُفْرٌ } مُتَّفَقٌ عَلَيْهِ. [1] .
Ibn Mas'ud (RAA) narrated that the Messenger of Allah (ﷺ) said: insulting a Muslim is disobedience to Allah, and fighting with him is Kufr (disbelief).” Agreed upon.
Arabic/English book reference: Book 16, Hadith 1530