Kural - 811

Though evil men should all-absorbing friendship show,
Their love had better die away than grow

Explanation
The decrease of friendship with those who look as if they would eat you up (through excess of love) while they are really destitute of goodness is far better than its increase.

Tamil Transliteration
Parukuvaar Polinum Panpilaar Kenmai
Perukalir Kundral Inidhu.