The Prophet, said, "Abusing a Muslim is Fusuq (evil doing) and killing him is Kufr (disbelief).

قَالَ النَّبِيُّ ﷺ سِبَابُ الْمُسْلِمِ فُسُوقٌ وَقِتَالُهُ كُفْرٌ