Qur'an, Al-Hujurat, Surah 49:10-11
The believers among themselves are like brothers. Therefore make peace between your brothers, and observe your duty to Allah that you may obtain mercy. Believers, let no one mock the other, for it could very well be that they are better than yourselves, nor let women mock other women, neither defaming nor insulting one another by nicknames. Accusation of depravity after faith has been accepted is a great evil indeed. Those who do not repent are the actual transgressors.