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 فُسُوقٌوَقِتَالُهُ كُفْرٌ قَالَ قُلْتُ لِأَبِي وَائِلٍ أَنْتَ سَمِعْتَ مِنْ عَبْدِ اللهِ قَالَ نَعَمْ