(37) The Book of Fighting [The Prohibition of Bloodshed]
كتاب تحريم الدم
(27) Chapter: Fighting Muslims
(27) باب قِتَالِ الْمُسْلِمِ ‏‏
Sunan an-Nasa'i 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 ‏.‏
Grade :  Sahih (Darussalam)
Reference : Sunan an-Nasa'i 4112
In-book reference : Book 37, Hadith 147
English translation : Vol. 5, Book 37, Hadith 4117