The Book of Fighting [The Prohibition of Bloodshed]
37/27. Chapter:
Fighting Muslims
Sunan an-Nasai
أَخْبَرَنَا مُحَمَّدُ بْنُ الْعَلاَءِ، عَنْ أَبِي مُعَاوِيَةَ، عَنِ الأَعْمَشِ، عَنْ شَقِيقٍ، عَنْ عَبْدِ اللَّهِ، قَالَ قِتَالُ الْمُؤْمِنِ كُفْرٌ وَسِبَابُهُ فُسُوقٌ .
It was narrated that 'Abdullah said: "Fighting a believer is Kufr and defaming him is evildoing."
Reference : Sunan an-Nasa'i 4113In-book reference : Book 37, Hadith 148English translation : Vol. 5, Book 37, Hadith 4118