Abusing a Muslim is an outrage and fighting against him is unbelief. Zubaid said: I asked Abu Wa'il: Did you hear it from Abdullah narrating if from the Messenger of Allah (may peace and blessings be upon him)? He replied: Yes. But there is mention of the talk between Zubaid and Abu Wa'il in the hadith narrated by Shu'ba.  

قَالَ رَسُولُ اللَّهِ ﷺ سِبَابُ الْمُسْلِمِ فُسُوقٌ وَقِتَالُهُ كُفْرٌ قَالَ زُبَيْدٌ فَقُلْتُ لأَبِي وَائِلٍ أَنْتَ سَمِعْتَهُ مِنْ عَبْدِ اللَّهِ يَرْوِيهِ عَنْ رَسُولِ اللَّهِ ﷺ قَالَ نَعَمْ وَلَيْسَ فِي حَدِيثِ شُعْبَةَ قَوْلُ زُبَيْدٍ لأَبِي وَائِلٍ  

Abu Bakr b. Abu Shaiba narrated a hadith like this from the Apostle (may peace and blessings be upon him) on the authority of Abdullah.  

عَنِ النَّبِيِّ ﷺ بِمِثْلِهِ