O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin. And do not spy or backbite each other. Would one of you like to eat the flesh of his brother when dead? You would detest it. And fear Allah ; indeed, Allah is Accepting of repentance and Merciful.
The following hadith explains what backbiting is.
Abu Hurairah (May Allah be pleased with him) said:
The Messenger of Allah (sallallaahu alayhi wa sallam) said, "Do you know what is backbiting?"
The Companions said: "Allah and His Messenger know better."
Thereupon he said, "Backbiting is talking about your (Muslim) brother in a manner which he dislikes."
It was said to him: "What if my (Muslim) brother is as I say."
He said, "If he is actually as you say, then that is backbiting; but if that is not in him, that is slandering."
Concentrate on the line in blue. That is what backbiting is. When you speak behind your brother's back and say something which is true.
People try to justify their backbiting by saying but what I said is true.
Well, that is what backbiting is. Had it been untrue, it would have been slander.
There are exceptions to when something is not considered backbiting. But for the normal individual he needs not concern himself with that until he first learns to control his tongue.
Stop backbiting your brother(s) in every gathering you attend.