Shemiras Halashon Halachos
Hilchos Lashon Hara chapter 9, section 1 The Law regarding Dust of Lashon Hara
There are things forbidden as 'dust' of lashon hara. For example to say, "Who would have thought that so and so would turn out as he is now," or, "Don't speak about so and so. I don't want to say what happened." Also, speaking praise of someone before his enemies as it causes them to speak disparagingly of him. One may also not praise a person excessively in general because at the end he will generally conclude, "[He is wonderful] except for this one thing about him." Or the listeners will respond, "Why are you praising him so much? Doesn't he have this certain [negative] quality?"
Hilchos Lashon Hara chapter 9, section 2 Before many people, one may not even praise him a little because there will generally be those who agree or disagree or are jealous of him, and will disparage him because of the praise. If the person spoken of is known to be righteous and without evil, one may praise him even before an enemy or jealous people as they will not be able to disparage him, and if they do, everyone will know they are lying.