"The Messenger of Allah ﷺ said: 'Verbally abusing a Muslim is immorality and fighting him is Kufr (disbelief).'" (Using translation from Ibn Mājah 69)

سَمِعْتُ رَسُولَ اللهِ ﷺ يَقُولُ «سِبَابُ الْمُسْلِمِ فُسُوقٌ وَقِتَالُهُ كُفْرٌ»