“Verbally abusing a Muslim is Fusuq (disobedience) and fighting him is Kufr (ungratefulness to Allah).” (Using translation from Ibn Mājah 3940)

يُحَدِّثُهُ عَنِ النَّبِيِّ ﷺ قَالَ سِبَابُالْمُسْلِمِ فُسُوقٌ وَقِتَالُهُ كُفْرٌ