Sayyiduna Imran bin Husain (may Allah be pleased with him) narrates that the Prophet (peace and blessings of Allah be upon him) said: "When a man says to his brother: 'O disbeliever!' then (the sin of) it is equivalent to killing him, and cursing a believer is also like killing him."

Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrates that the Messenger of Allah (peace and blessings be upon him) said: "If a man calls his brother a disbeliever, then one of them will certainly become a disbeliever. If the person is truly a disbeliever, then it is correct; otherwise, this description will return to the one who said it."