Kural - 182

Than he who virtue scorns, and evil deeds performs, more vile,
Is he that slanders friend, then meets him with false smile

Explanation
To smile deceitfully (in another's presence) after having reviled him to his destruction (behind his back) is a greater evil than the commission of (every other) sin and the destruction of (every) virtue.

Tamil Transliteration
Aranazheei Allavai Seydhalin Theedhe
Puranazheeip Poiththu Nakai.