Sunan Ibn Majah, Hadith 3939 ⓘ
It was narrated from Ibn Mas’ud that the Messenger of Allah said: “Verbally abusing a Muslim is Fusuq disobedience and fighting him is Kufr (ungratefulness to Allah).’”
حَدَّثَنَا 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 " .
also referenced under: Book 36, Hadith 14 (In-Book) & Volume 5, Book 36, Hadith 3939 (DarusSalam)