Qur'an, Al-Hujurat, Surah 49:11
Believers, let no man among you mock another man; the man being mocked is usually superior to the man who mocks. Let no woman among you mock another woman; the woman being mocked is usually superior to the woman who mocks. Do not slander one another, nor call each other by rude names; it is wrong that someone who has embraced the faith, should be called by a rude name. Those who do not repent, are sinners.