Verbally abusing a Muslim is Fusuq (disobedience) and fighting Him is Kufr (ungratefulness to Allah)
حَدَّثَنَا عَلِيُّ بْنُ مُحَمَّدٍ، حَدَّثَنَا وَكِيعٌ، عَنْ شَرِيكٍ، عَنْ أَبِي إِسْحَاقَ، عَنْ مُحَمَّدِ بْنِ سَعْدٍ، عَنْ سَعْدٍ، قَالَ قَالَ رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ " سِبَابُ الْمُسْلِمِ فُسُوقٌ وَقِتَالُهُ كُفْرٌ " .
It was narrated from Sa’d that the Messenger of Allah (ﷺ) said: “Verbally abusing a Muslim is Fusuq (disobedience) and fighting him is Kufr.”