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