Kural - 293

Speak not a word which false thy own heart knows
Self-kindled fire within the false one's spirit glows

Explanation
Let not a man knowingly tell a lie; for after he has told the lie, his mind will burn him (with the memory of his guilt).

Tamil Transliteration
Thannenj Charivadhu Poiyarka Poiththapin
Thannenje Thannaich Chutum.