Narrator:Abdullah ibn Mas‘ūd
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.'"
Reported by An-Nasa’i (No. 4111, 4105, 4107, 4108, 4109, 4110, 4112), 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).