Narrated by Abu Huraira (Radhiallaho anho):
Allah's Messenger (sallAllaahu `alaihi wasallam) said: : Do you know
what is backbiting? They (the Companions) said: Allah and His Apostle
(peace be upon him) know best. Thereupon he (the Prophet) said:
Backbiting implies your talking about your brother in a manner which
he does not like. It was said to him: What is your opinion about this
that if I actually find (that failing) in my brother which I made a
mention of? He said: If (that failing) is actually found (in him)
what you assert, you in fact backbited him, and if that is not in him
it is a slander.