وَالَّذِيۡنَ يُؤۡذُوۡنَ الۡمُؤۡمِنِيۡنَ وَالۡمُؤۡمِنٰتِ بِغَيۡرِ مَا اكۡتَسَبُوۡا فَقَدِ احۡتَمَلُوۡا بُهۡتَانًا وَّاِثۡمًا مُّبِيۡنًا
Al-Ahzab (33:58)
Those who cause hurt to believing men and to believing women have invited upon themselves a calumny and a manifest sin.
This verse determines the definition of slander. It is to ascribe a fault to a person which he does not have, or an error which he has not committed. The Prophet (peace be upon him) also has explained it. According to Abu Daud and Tirmidhi, when he was asked as to what is ghibat (backbiting), he replied: “It is to make mention of your brother in a manner derogatory to him.” The questioner said, “And if the fault is there in my brother?” The Prophet (peace be upon him) replied: “If the fault that you mentioned is there in him, you backbite him; if it is not there, you slandered him.” Such an act is not only a moral sin, which will entail punishment in the Hereafter, but this verse also requires that also in the law of an Islamic State false allegation should be held as a culpable offense.
http://islamicstudies.info/tafheem.php?sura=33&verse=53&to=58