صحيح مسلم
Sahih Muslim
كتاب الإيمان
The Book of Faith
باب بَيَانِ قَوْلِ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «سِبَابُ الْمُسْلِمِ فُسُوقٌ وَقِتَالُهُ كُفْرٌ»:
Chapter: Clarifying the words of the prophet (saws): "Insulting A Muslim is an evil action and fighting him is disbelief (Kufr)"
2 hadith
Hadith 64
It is narrated on the authority of 'Abdullah bin Mas'ud that the Messenger of Allah (ﷺ) (may peace and blessings be upon him) observed: 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 (ﷺ) (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'bah.
Hadith 64
Abu Bakr (RA) bin Abu Shaiba narrated a hadith like this from the Apostle (ﷺ) (may peace and blessings be upon him) on the authority of 'Abdullah.