Kural - 188

Whose nature bids them faults of closest friends proclaim
What mercy will they show to other men's good name

Explanation
What will those not do to strangers whose nature leads them to publish abroad the faults of their intimate friends ?.

Tamil Transliteration
Thunniyaar Kutramum Thootrum Marapinaar
Ennaikol Edhilaar Maattu.