Sunan an-Nasa'i, Hadith 4112 ⓘ
It was narrated that Abu Wa'il said: "'Abdullah said: 'Defaming a Muslim is evildoing and fighting him is Kufr.'"
أَخْبَرَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، قَالَ حَدَّثَنَا جَرِيرٌ، عَنْ مَنْصُورٍ، عَنْ أَبِي وَائِلٍ، قَالَ قَالَ عَبْدُ اللَّهِ سِبَابُ الْمُسْلِمِ فُسُوقٌ وَقِتَالُهُ كُفْرٌ .
also referenced under: Book 37, Hadith 147 (In-Book) & Volume 5, Book 37, Hadith 4117 (DarusSalam)