Sunan Ibn Majah, Book 36, Hadith 14 (In-Book) ⓘ
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: Hadith 3939 (Reference) & Volume 5, Book 36, Hadith 3939 (DarusSalam)