Backbiting (Gheebah) means speaking about a Muslim in his\her absence and saying things that he\she would not like to have spread around or mentioned.   Slander means saying things about a Muslim that are not true, or in other words telling lies.  Gossip means telling a person what another said in order to cause trouble between them. Islam Strongly prohibits all three and there are a lot of evidence through Quran and Hadith to show that these actions are Haram and Muslims should avoid them.

AllahSWT says in the Holy Quran: "And backbite not one another. Would one of you like to eat the flesh of his dead brother? You would hate it (so hate backbiting). And fear Allah. Verily, Allah is the One Who forgives and accepts repentance, Most Merciful.'' (49:12)

Prophet PBUH said, “Backbiting is to say something about your brother that he would dislike."  Someone then asked him, "But what if what I say is true?" He PBUH replied, "If what you say about him is true, you are backbiting him, but if it is not true then you have slandered him".

Prophet PBUH said, "No fire is faster in consuming dry wood than backbiting is in consuming a devotee's virtues."

Prophet PBUH said: "He who believes in Allah and the Last Day must either speak good or remain silent.''

Prophet PBUH said, “A man might speak a word without thinking about its implications, but because of it, he will plunge into the Hellfire further than the distance between the east and west”.

Prophet PBUH was once asked, “Who is the best Muslim?”  He PBUH replied, "He is the one from whom Muslims are safe from the evil of his tongue and hands."

Prophet PBUH said, "The person who protects his tongue from unlawful utterances and his private parts from illegal sexual conduct, I shall guarantee him entrance into Paradise."

Prophet PBUH said: "When the son of Adam gets up in the morning, all the limbs humble themselves before the tongue and say: `Fear Allah for our sake because we are with you: (i.e. we will be rewarded or punished as a result of what you say) if you are straight, we will be straight; and if you are crooked, we will become crooked.''

Prophet PBUH once 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, and the other used not to take precautions to avoid getting urine on himself when he urinated.”