Silsilah as-Sahihah 2516 — Fighting a Muslim is disbelief, insulting

Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "For a Muslim to abuse his brother is wickedness, and to kill him is disbelief, and the sanctity of his wealth is like the sanctity of his blood."