Sunan an-Nasa'i 4106
سنن النسائي#4106Sahihأَخْبَرَنَا يَحْيَى بْنُ حَكِيمٍ، قَالَ حَدَّثَنَا عَبْدُ الرَّحْمَنِ بْنُ مَهْدِيٍّ، عَنْ شُعْبَةَ، عَنْ أَبِي إِسْحَاقَ، عَنْ أَبِي الأَحْوَصِ، عَنْ عَبْدِ اللَّهِ، قَالَ سِبَابُ الْمُسْلِمِ فِسْقٌ وَقِتَالُهُ كُفْرٌ . فَقَالَ لَهُ أَبَانُ يَا أَبَا إِسْحَاقَ أَمَا سَمِعْتَهُ إِلاَّ مِنْ أَبِي الأَحْوَصِ قَالَ بَلْ سَمِعْتُهُ مِنَ الأَسْوَدِ وَهُبَيْرَةَ .
It was narrated from Shu'bah, from Abu Ishaq who said:"I heard Abu Al-Ahwas (narrate) that 'Abdullah said: 'Defaming a Muslim is evildoing and fighting him is Kufr.'" So Aban said to him: "O Abu Ishaq! You heard it only from Abu Al-Ahwas?" He said: "Rather, I heard it from Al-Aswad and Hubairah
Scholarly Authentication
The label above is the grade most of these scholars gave — one dissenting opinion no longer speaks for all of them. If any scholar calls a narration fabricated, that verdict is shown instead, whatever the others say. Each scholar's own wording:
In-book reference: 37:141