Mukhtasar Muslim (Abridged) 65 — Abusing a Muslim is sin; fighting him

`Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) reports that` the Messenger of Allah (peace and blessings be upon him) said: To abuse (or mention the faults of) a Muslim is wickedness (i.e., it is a sin and the one who does so becomes a transgressor), and fighting him is disbelief.