(4) باب سِبَابُ الْمُسْلِمِ فُسُوقٌ وَقِتَالُهُ كُفْرٌ
It was narrated from Ibn Mas'ud that the Messenger of Allah (ﷺ)said:
"Verbally abusing a Muslim is Fusuq disobedience and fightinghim is Kufr (ungratefulness to Allah).'"
Reference: Sunan Ibn Majah 3939
In-book reference: Book 36, Hadith 14
English translation: Vol. 5, Book 36, Hadith 3939