Sunan Ibn Majah 3939
سنن ابن ماجه#3939Sahihحَدَّثَنَا هِشَامُ بْنُ عَمَّارٍ، حَدَّثَنَا عِيسَى بْنُ يُونُسَ، حَدَّثَنَا الأَعْمَشُ، عَنْ شَقِيقٍ، عَنِ ابْنِ مَسْعُودٍ، قَالَ قَالَ رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ " سِبَابُ الْمُسْلِمِ فُسُوقٌ وَقِتَالُهُ كُفْرٌ " .
It was narrated from Ibn Mas’ud that the Messenger of Allah (ﷺ) said:“Verbally abusing a Muslim is Fusuq disobedience and fighting him is Kufr (ungratefulness to Allah).’”
Scholarly Authentication
The label above is the grade most of these scholars gave — one dissenting opinion no longer speaks for all of them. If any scholar calls a narration fabricated, that verdict is shown instead, whatever the others say. Each scholar's own wording:
Al-AlbaniSahih
Muhammad Fouad Abd al-BaqiSahih
Zubair Ali ZaiSahih - Agreed Upon
In-book reference: 36:14