Sunan an-Nasa'i, Volume 5, Book 37, Hadith 4116 (DarusSalam) ⓘ
It was narrated that 'Abdullah said: "The Messenger of Allah [SAW] said: 'Defaming a Muslim is evildoing and fighting him is Kufr.'"
أَخْبَرَنَا مَحْمُودُ بْنُ غَيْلاَنَ، قَالَ حَدَّثَنَا مُعَاوِيَةُ، قَالَ حَدَّثَنَا سُفْيَانُ، عَنْ مَنْصُورٍ، عَنْ أَبِي وَائِلٍ، عَنْ عَبْدِ اللَّهِ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " سِبَابُ الْمُسْلِمِ فُسُوقٌ وَقِتَالُهُ كُفْرٌ " .
also referenced under: Hadith 4111 (Reference) & Book 37, Hadith 146 (In-Book)