Kural - 830

When time shall come that foes as friends appear,
Then thou, to hide a hostile heart, a smiling face may'st wear

Explanation
When one's foes begin to affect friendship, one should love them with one's looks, and, cherishing no love in the heart, give up (even the former).

Tamil Transliteration
Pakainatpaam Kaalam Varungaal Mukanattu
Akanatpu Oreei Vital.