The Book of Fighting [The Prohibition of Bloodshed]
37/27. Chapter:
Fighting Muslims
Sunan an-Nasai
أَخْبَرَنَا مَحْمُودُ بْنُ غَيْلاَنَ، قَالَ حَدَّثَنَا مُعَاوِيَةُ، قَالَ حَدَّثَنَا سُفْيَانُ، عَنْ مَنْصُورٍ، عَنْ أَبِي وَائِلٍ، عَنْ عَبْدِ اللَّهِ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " سِبَابُ الْمُسْلِمِ فُسُوقٌ وَقِتَالُهُ كُفْرٌ " .
It was narrated that 'Abdullah said: "The Messenger of Allah [SAW] said: 'Defaming a Muslim is evildoing and fighting him is Kufr.'"
Reference : Sunan an-Nasa'i 4111In-book reference : Book 37, Hadith 146English translation : Vol. 5, Book 37, Hadith 4116