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