Sunan an-Nasa'i, Book 37, Hadith 148 (In-Book) ⓘ
It was narrated that 'Abdullah said: "Fighting a believer is Kufr and defaming him is evildoing."
أَخْبَرَنَا مُحَمَّدُ بْنُ الْعَلاَءِ، عَنْ أَبِي مُعَاوِيَةَ، عَنِ الأَعْمَشِ، عَنْ شَقِيقٍ، عَنْ عَبْدِ اللَّهِ، قَالَ قِتَالُ الْمُؤْمِنِ كُفْرٌ وَسِبَابُهُ فُسُوقٌ .
also referenced under: Hadith 4113 (Reference) & Volume 5, Book 37, Hadith 4118 (DarusSalam)