Sunan Abi Dawud — Model Behavior of the Prophet (Kitab Al-Sunnah) — Hadith No: 4687
Translation
Ibn ‘Umar reported the Messenger of Allah (May peace be upon him) as saying :If any believing man calls another believing man an unbeliever, if he is actually an infidel, it is all right ; if not, he will become an infidel