Once two people were backbiting about a person who had committed adultery. Prophet Muhammad (saw) heard their conversation and kept on walking quietly. After a while, they came across a dead carcass of a donkey with scattered legs. Rasoolallah (Sallallahu Alaihi Wasallam) said to them, "Go and eat the flesh of this dead donkey." They said, "How can the flesh of a dead donkey be eaten?" Rasoolallah (Sallallahu Alaihi Wasallam) said, "You insulted your brother, which is worse than eating the flesh of a dead donkey."
The above incident has been extracted from a hadith narrated by Abu Dawud.
We learn from this incident that the foul effect of backbiting is even worse than eating the meat of a dead rotten animal. Backbiting is sweet to the tongue but it is similar to smoking. The smoker enjoys when he is smoking but at the same time, he is destroying his lungs. In the same manner, when a person backbites, he/she is destroying their own good deeds by giving them away to the person they are talking about.
Moreover, if someone backbites a lot, do not trust them with your secrets. That's because, if they can talk about other people without any hesitation, they can certainly talk about you with others.