Sayyiduna Imran bin Husayn (may Allah be pleased with him) narrates that the Prophet (peace and blessings be upon him) said: "When a man says to his brother, 'O disbeliever!', it is like killing him."
Al-Bazzar recorded it, and its narrators are thiqat (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الأدب / 13016