Sunan Ibn Majah, Volume 5, Book 36, Hadith 3940 (DarusSalam) ⓘ
It was narrated from Abu Hurairah that the Prophet said: “Verbally abusing a Muslim is Fusuq (disobedience) and fighting him is Kufr (ungratefulness to Allah).”
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا مُحَمَّدُ بْنُ الْحَسَنِ الأَسَدِيُّ، حَدَّثَنَا أَبُو هِلاَلٍ، عَنِ ابْنِ سِيرِينَ، عَنْ أَبِي هُرَيْرَةَ، عَنِ النَّبِيِّ ـ صلى الله عليه وسلم ـ قَالَ " سِبَابُ الْمُسْلِمِ فُسُوقٌ وَقِتَالُهُ كُفْرٌ " .
also referenced under: Hadith 3940 (Reference) & Book 36, Hadith 15 (In-Book)