"‘Abdullāh ibn Mas‘ūd reported that the Prophet (may Allah's peace and blessings be upon him) said: ""Reviling a Muslim is disobedience and fighting him is disbelief."" [Al-Bukhari]

"عن عبد الله بن مسعود -رضي الله عنه- عن النبي -صلى الله عليه وسلم- قال: «سِبابُ المسلم فسوق, وقتاله كفر». [صحيح.] - [متفق عليه.]"