(4) باب سِبَابُ الْمُسْلِمِ فُسُوقٌ وَقِتَالُهُ كُفْرٌ
It was narrated from Abu Hurairah that the Prophet (ﷺ) said:
"Verbally abusing a Muslim is Fusuq (disobedience) and fighting himisKufr (ungratefulness to Allah)."
Reference: Sunan Ibn Majah 3940
In-book reference: Book 36, Hadith 15
English translation: Vol. 5, Book 36, Hadith 3940