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