It was narrated that 'Abdullah said:
"The Messenger of Allah (ﷺ) said: 'Verbally abusing a Muslim is immorality and fighting him is Kufr (disbelief).'" (PT)
English reference (PT): Vol. 1, Book 1, Hadith 69 (PT)
Arabic reference (PT): Book 1, Hadith 72 (PT)