Narrator:Abdullah ibn Mas‘ūd
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.'"
Reported by An-Nasa’i (No. 4112, 4105, 4107, 4108, 4109, 4110, 4111), Al-Bukhari (No. 48, 6044, 7076), Muslim (No. 64a), At-Tirmidhi (No. 1983, 2635), Ibn Majah (No. 69, 3939) and Ahmad (No. 3465, 3708, 3916, 4115).