It is narrated from Sayyiduna Abdullah bin Amir (may Allah be pleased with him), he said: O Abu Mas’ud! What have you heard from the Messenger of Allah (peace and blessings of Allah be upon him) about the word “za’amu” (they claim)? He said: I heard him (peace and blessings of Allah be upon him) say: “This word is the worst mount of a man.” And I heard him (peace and blessings of Allah be upon him) say: “Cursing a believer is like killing him.”
Benefits and Issues: In this narration, the reprehensibility of cursing a Muslim is mentioned: that if someone is cursed and he is not deserving of it, then its sin is equal to that of killing.
Source: Fadlullah al-Ahad: Urdu Commentary on al-Adab al-Mufrad, Page: 763