The Prophet (salAllahu alayhi wasalam) was sitting with his companions one day and one of them was speaking badly about someone who wasn't there. As the man got up to leave the Prophet (salAllahu alayhi wasalam) said to him: "Pick Your Teeth!" "But I haven't eaten anything," the man protested. "No," the Prophet (salAllahu alayhi wasalam) said "You have eaten the flesh of your dead brother".
As Allah tells us in the Qur'an:
"Would any of you like to eat the flesh of his dead brother? No, you would hate it."
[surah al-Hujurat; 49:12]
But What If It's True?!
The Prophet (salAllahu alayhi wasalam) told us:
"Backbiting is to say something about someone they wouldn't like said about them....If what bad you said about them is true, then you have backbited and if it is false then you have slandered them."
If you're still not convinced of how big a sin backbiting really is, then look at the punishment Allah has in store for the backbiters. The Prophet (salAllahu alayhi wasalam) described:
"On the night of mirage I passed by some people who had metal hooks in their hands and were clawing at their faces and their necks with them. I asked Gabriel 'Who were these people?' He said 'These are the people who eat the flesh of human beings and disgrace them'
This is how big a sin backbiting is looked upon and yet we continue to backbite without a second thought. Next time you lie, talk about somebody, remember Allah (subhanahu wa Ta'ala) and the Prophet (salAllahu alayhi wasalam) and put them in front of you before you speak