Sahih Bukhari Volume 008, Book 073, Hadith Number 073.

Narated By Thabit bin Ad-Dahhak : (Who was one of the companions who gave the pledge of allegiance to the Prophet underneath the tree (Al-Hudaibiya)) Allah's Apostle said, "Whoever swears by a religion other than Islam (i.e. if somebody swears by saying that he is a non-Muslim e.g., a Jew or a Christian, etc.) in case he is telling a lie, he is really so if his oath is false, and a person is not bound to fulfill a vow about a thing which he does not possess. And if somebody commits suicide with anything in this world, he will be tortured with that very thing on the Day of Resurrection; And if somebody curses a believer, then his sin will be as if he murdered him; And whoever accuses a believer of Kufr (disbelief), then it is as if he killed him."