Allaah says (interpretation of the meaning):
“neither backbite one another. Would one of you like to eat the flesh of his dead brother? You would hate it (so hate backbiting). And fear Allaah. Verily, Allaah is the One Who forgives and accepts repentance, Most Merciful”
[al-Hujuraat 49:12]
It was narrated from Abu Hurayrah (may Allaah be pleased with him) that the Messenger of Allaah SAWS (peace and blessings of Allaah be upon him) said: “Do you know what gheebah (backbiting) is?” They said, “Allaah and His Messenger know best.” He said, “Saying something about your brother that he dislikes.” It was said, “What if what I say about my brother is true?” He said, “If what you say is true then you have backbitten about him, and if it is not true, then you have slandered him.”
Narrated by Muslim, 2589
It was narrated that Ibn ‘Abbaas said: The Messenger of Allaah (peace and blessings of Allaah be upon him) passed by two graves and said, “They are being punished, but they are not being punished for anything that was difficult to avoid. One of them used to walk about spreading malicious gossip (nameemah), and the other used not to take precautions to avoid getting urine on himself when he urinated.” Then he called for a green branch, which he split in two and planted a piece on each grave, and said, “May their torment be reduced so long as these do not dry out.”
Narrated by al-Bukhaari, 213; Muslim, 292
For a person to say of another, “He cannot control his tongue (or he has a loose tongue)” is undoubtedly one of those things that a person would dislike to have said about him. If it is true, then it is gheebah (backbiting), and if it is not true then it is buhtaan (slander).