Abu Huraira reported Allah's Messenger (ﷺ) as saying: Do you know what is backbiting?
They (the Companions) said: Allah and His Messenger know best. Thereupon he the Holy Prophet (ﷺ) 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 (ﷺ) 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.
Backbiting is forbidden
Allah forbids these acts, as He says in the Holy Quran: "Backbiting and gossiping are from the vilest and despicable of things, yet the most widely spread amongst mankind, such that no one is free from it except for a few people." (Quran)