In The Name of Allaah, The Most Merciful, The Bestower of Mercy.
Abu Hurairah [radiyallaahu-anhu] narrated that the Messenger of Allaah [sallal-laahu-alayhi-wasallam] said, ‘’Do you know what backbiting is?’’ They (i.e. the sahaabah) replied Allaah and his Messenger know best’’; he said [sallal-laahu-alayhi-wasallam] said, ‘’It is to mention your brother what he dislikes’’. Someone asked, ‘’What if what I said is true?’’ He [sallal-laahu-alayhi-wasallam] replied, ‘’If what you said about him is true you have backbitten him, and if it is not true you have slandered him’’. [Muslim]
When is speaking about someone behind his back not considered backbiting? It is not considered backbiting when a person makes a complaint about an oppressor to a person who has the ability to stop the oppression; when seeking help to stop an evil- by making that known to the person who has the ability to stop the evil; when seeking for a fatwa; when warning the Muslims so that they are not deceived; when mentioning a person who commits his evil openly or one who calls to bidah and when identifying someone with a physical defect, whilst not intending defamation. [Subul As-Salaam- Vol 4 page 553-554]