3: 118 O ye who believe! take not into your intimacy those outside your ranks; they will not fail to corrupt you. They only desire your ruin: rank hatred has already appeared from their mouths; what their hearts conceal is far worse. We have made plain to you the Signs if ye have wisdom.
If you speak of a person behind their back and that evil is found in him even then you are guilty of backbiting. (Abu Dawood & Tirmizi)