Abu Hurairah (Radiyallaahu `anhu) reported Allah’s last Messenger (Sallallaahu `alayhi wa sallam) said:

“Do you know what is backbiting?”

They (the Companions) said:

“Allah and His Messenger 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.”

Sahih Muslim (Book 032, Number 6265)