Anything secretly whispered of an absent person which is calculated to injure him, and which is true, is called Ghibah a false accusation being expressed by Buhtan. Abu Hurairah says, “The question was put to the Prophet, ‘Do you know what backbiting is?'” and he replied, “It is saying anything bad of a Muslim.” It was then said, ‘But what if it is true?’
And he said: “If it is true it is Ghibah, and if it is a false accusation, it is Buhtan (i.e, ‘slander'” (Mishkat, xxii. c. x.)
The following are sayings of Muhammad on the subject: – “The best of God’s servants are those who when you meet them speak of God. The worst of God’s servants are those who carry tales about, to do mischief and separate friends, and seek out the defects of good people.” “He who wears two faces in this world shall have two tongues of fire in the day of the resurrection.” “It is unworthy of a believer to injure people’s reputations, or to curse anyone, or to abuse anyone, or to talk vainly.” “The best atonement you can make for backbiting is to say, ‘O God pardon me and him (whom I have injured).'” Mishkat, xxii. c.x.
Based on Hughes, Dictionary of Islam