Silsilah as-Sahihah 93 — Caution in declaring someone a disbeliever

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."