Kural - 291

You ask, in lips of men what 'truth' may be;
'Tis speech from every taint of evil free

Explanation
Truth is the speaking of such words as are free from the least degree of evil (to others).

Tamil Transliteration
Vaaimai Enappatuvadhu Yaadhenin Yaadhondrum
Theemai Ilaadha Solal.