Bukhari

Narrated Abu Huraira: Allah's Apostle said, "If a man says to his brother, O Kafir (disbeliever)!' Then surely one of them is such (i.e., a Kafir). "

Narrated 'Abdullah bin 'Umar: Allah's Apostle said, 'If anyone says to his brother, 'O misbeliever! Then surely, one of them such."

Narrated Thabit bin Ad-Dahhak: The Prophet said, "Whoever swears by a religion other than Islam (i.e. if he swears by saying that he is a non-Muslim in case he is telling a lie), then he is as he says if his oath is false and whoever commits suicide with something, will be punished with the same thing in the (Hell) fire, and cursing a believer is like murdering him, and whoever accuses a believer of disbelief, then it is as if he had killed him."