Sunan an-Nasa'i
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