It is narrated from Sayyiduna Imran bin Husayn (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "When a man says to his brother, 'O disbeliever!', it is like killing him, and cursing a believer is like killing him."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 15554