سُنَنُ النَّسَائِيِّ
Sunan an-Nasa’i 4112
The Book of Fighting [The Prohibition of Bloodshed]
Fighting Muslims
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.'"
Sunan an-Nasa'i, 4112Sahih (Darussalam)