The Prophet ﷺ said, "Abusing a Muslim is Fusuq (an evil doing) and killing him is Kufr (disbelief)." (Using translation from Bukhārī 48)

عَنِ النَّبِيِّ ﷺ قَالَ «سِبَابُ الْمُسْلِمِ فِسْقٌ وَقِتَالُهُ كُفْرٌ»