"'Abdullah said: 'Defaming a Muslim is evildoing and fighting him is Kufr.'" (Using translation from Nasāʾī 4112)

قَالَ عَبْدُ اللهِ «سِبَابُ الْمُسْلِمِ فُسُوقٌ وَقِتَالُهُ كُفْرٌ»