Hilchos Lashon Hara, chapter 1, section 5

Regarding speaking lashon hara, it makes no difference whether the person spoke of his own volition, or if his fellow persuaded him to speak until he spoke. It is forbidden in every case. Even if his father or rav whom he is obligated to fear asked him to tell them something, and he knows that lashon hara is included in it or even the dust of lashon hara, he may not obey them.