NASAI 4108:
from ʿAbd al-Raḥman b. ʿAbdullāh from his father —
The Messenger of Allah [SAW] said: "Defaming a Muslim is evildoing and fighting him is Kufr."
النسائي ٤١٠٨:
عن عبد الرحمن بن عبد الله عن أبيه —
أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ سِبَابُ الْمُسْلِمِ فُسُوقٌ وَقِتَالُهُ كُفْرٌ