(4) باب سِبَابُ الْمُسْلِمِ فُسُوقٌ وَقِتَالُهُ كُفْرٌ
It was narrated from Sa'd that the Messenger of Allah (ﷺ) said:
"Verbally abusing a Muslim is Fusuq (disobedience) and fightinghimis Kufr."
Reference: Sunan Ibn Majah 3941
In-book reference: Book 36, Hadith 16
English translation: Vol. 5, Book 36, Hadith 3941