Sunan Ibn Majah
3939
Sahih+2

It was narrated from Ibn Mas’ud that the Messenger of Allah (ﷺ) said:“Verbally abusing a Muslim is Fusuq disobedience and fighting him is Kufr (ungratefulness to Allah).’”

SahihAl-Albani
SahihMuhammad Fouad Abd al-Baqi