3: 119 Ah! ye are those who love them but they love you not though ye believe in the whole of the Book. When they meet you they say "We believe"; but when they are alone they bite off the very tips of their fingers at you in their rage. Say: "Perish in your rage; Allah knoweth well all the secrets of the heart."
If you speak of a person behind their back and that evil is not found in them then you are guilty of slander. (Abu Dawood & Tirmizi)