The Book of Fighting [The Prohibition of Bloodshed]
37/27. Chapter:
Fighting Muslims
Sunan an-Nasai
أَخْبَرَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، قَالَ حَدَّثَنَا جَرِيرٌ، عَنْ مَنْصُورٍ، عَنْ أَبِي وَائِلٍ، قَالَ قَالَ عَبْدُ اللَّهِ سِبَابُ الْمُسْلِمِ فُسُوقٌ وَقِتَالُهُ كُفْرٌ .
It was narrated that Abu Wa'il said: "'Abdullah said: 'Defaming a Muslim is evildoing and fighting him is Kufr.'"
Reference : Sunan an-Nasa'i 4112In-book reference : Book 37, Hadith 147English translation : Vol. 5, Book 37, Hadith 4117