Do you know what is backbiting?
They (the Companions) said: Allah and His Messenger know best. Thereupon he (the Holy 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?
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.
MALIKS MUWATTA: Book 56, Number 56.4.10:
Malik related to me from al-Walid ibn Abdullah ibn Sayyad that al-Muttalib ibn Abdullah ibn Hantab al-Makhzumi informed him that a man asked the Messenger of Allah, may Allah bless him and grant him peace,
“What is backbiting?”
The Messenger of Allah, may Allah bless him and grant him peace, said,
“It is to mention about a man what he does not want to hear.”
He said, “Messenger of Allah! Even if it is true?” The Messenger of Allah, may Allah bless him and grant him peace, said,
“If you utter something false, then it is slander.”