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