The Prophet ﷺ said, "Abusing a Muslim is Fusuq (an evil doing) and killing him is Kufr (disbelief)." (Using translation from Bukhārī 48)
عَنِ النَّبِيِّ ﷺ قَالَ سِبَابُ الْمُسْلِمِ فُسُوقٌوَقِتَالُهُ كُفْرٌ قَالَ قُلْتُ لِأَبِي وَائِلٍ أَنْتَ سَمِعْتَ مِنْ عَبْدِ اللهِ قَالَ نَعَمْ
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
"The Messenger of Allah [SAW] 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."
قَالَ رَسُولُ اللَّهِ ﷺ سِبَابُ الْمُسْلِمِ فُسُوقٌ وَقِتَالُهُ كُفْرٌ قُلْتُ لأَبِي وَائِلٍ سَمِعْتَهُ مِنْ عَبْدِ اللَّهِ قَالَ نَعَمْ
"Verbally abusing the Muslim is disobedience and fighting him is disbelief." Zubaid said:"I said to Abu Wa'il: 'Did you hear it from 'Abdullah?' He said: 'Yes.'"
قَالَ رَسُولُ اللَّهِ ﷺ سِبَابُ الْمُسْلِمِ فُسُوقٌ وَقِتَالُهُ كُفْرٌ قَالَ زُبَيْدٌ قُلْتُ لأَبِي وَائِلٍ أَأَنْتَ سَمِعْتَهُ مِنْ عَبْدِ اللَّهِ قَالَ نَعَمْ
[Machine] Zubayd, Mansur, and Sulaiman told me that they heard Abu Wail narrating from Abdullah, who narrated from the Prophet ﷺ, saying, "Insulting a Muslim is wickedness, and fighting against him is disbelief." Zubayd said, "I asked Abu Wail twice, 'Did you hear it from Abdullah, who narrated it from the Prophet ﷺ?' He replied, 'Yes.'"
عَنْ زُبَيْدٍ وَمَنْصُورٍ وَسُلَيْمَانَ أَخْبَرُونِي أَنَّهُمْ سَمِعُوا أَبَا وَائِلٍ يُحَدِّثُعَنْ عَبْدِ اللهِ عَنِ النَّبِيِّ ﷺ قَالَ سِبَابُ الْمُسْلِمِ فُسُوقٌ وَقِتَالُهُ كُفْرٌ قَالَ زُبَيْدٌ قُلْتُ لِأَبِي وَائِلٍ مَرَّتَيْنِ أَأَنْتَ سَمِعْتَهُ مِنْ عَبْدِ اللهِ عَنِ النَّبِيِّ ﷺ قَالَ نَعَمْ
[Machine] Zubaid, Mansour, and Sulaiman informed me that they heard Abu Wael narrating from Abdullah, who heard from the Prophet ﷺ saying, "Insulting a Muslim is immorality, and fighting him is disbelief." Zubaid then said, "I asked Abu Wael twice, 'Did you hear this from Abdullah, who heard it from the Prophet ﷺ ?' He replied, 'Yes.'"
زُبَيْدٌ وَمَنْصُورٌ وَسُلَيْمَانُ أَخْبَرُونِي أَنَّهُمْ سَمِعُوا أَبَا وَائِلٍ يُحَدِّثُ عَنْ عَبْدِ اللهِ عَنِ النَّبِيِّ ﷺ قَالَ سِبَابُ الْمُسْلِمِ فُسُوقٌ وَقِتَالُهُ كُفْرٌ قَالَ زُبَيْدٌ فَقُلْتُ لِأَبِي وَائِلٍ مَرَّتَيْنِ أَأَنْتَ سَمِعْتَهُ مِنْ عَبْدِ اللهِ عَنِ النَّبِيِّ ﷺ؟ قَالَ نَعَمْ
[Machine] On the authority of the Prophet ﷺ, he said, "Reviling a believer is an act of immorality (sin), and fighting against him is an act of disbelief (kufr)." In another hadith of Zubayd, I heard Aba Wa'il say.
عَنِ النَّبِيِّ ﷺ أَنَّهُ قَالَ سِبَابُ الْمُؤْمِنِ فِسْقٌ وَقِتَالُهُ كُفْرٌ قَالَ فِي حَدِيثِ زُبَيْدٍ سَمِعْتُ أَبَا وَائِلٍ
"The Messenger of Allah [SAW] 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." (Using translation from Nasāʾī 4110)
قَالَ رَسُولُ اللهِ ﷺ «سِبَابُ الْمُسْلِمِ فُسُوقٌ وَقِتَالُهُ كُفْرٌ» قُلْتُ لِأَبِي وَائِلٍ أَنْتَ سَمِعْتَهُ مِنْ عَبْدِ اللهِ؟ قَالَ نَعَمْ
"'Abdullah said: 'Defaming a Muslim is evildoing and fighting him is Kufr.'" (Using translation from Nasāʾī 4112)
قَالَ عَبْدُ اللهِ «سِبَابُ الْمُسْلِمِ فُسُوقٌ وَقِتَالُهُ كُفْرٌ»