Kural - 312

Though malice work its worst, planning no ill return, to endure,
And work no ill, is fixed decree of men in spirit pure

Explanation
It is the determination of the spotless not to do evil, even in return, to those who have cherished enmity and done them evil.

Tamil Transliteration
Karuththuinnaa Seydhavak Kannum Maruththinnaa
Seyyaamai Maasatraar Kol.