Sunan an-Nasai 18856
In-book no 4120أَخْبَرَنَا مَحْمُودُ بْنُ غَيْلاَنَ، قَالَ حَدَّثَنَا وَكِيعٌ، قَالَ حَدَّثَنَا سُفْيَانُ، عَنْ زُبَيْدٍ، عَنْ أَبِي وَائِلٍ، عَنْ عَبْدِ اللَّهِ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " سِبَابُ الْمُسْلِمِ فُسُوقٌ وَقِتَالُهُ كُفْرٌ " . قُلْتُ لأَبِي وَائِلٍ سَمِعْتَهُ مِنْ عَبْدِ اللَّهِ قَالَ نَعَمْ .
It was narrated from Sufyan bin Zubaid, from Abu Wa'il, from 'Abdullah: "The Messenger of Allah said: 'Defaming a Muslim is evildoing and fighting him is Kufr.'" I said to Abu Wa'il: "Did you hear it from 'Abdullah?" He said: "Yes."