Kural - 139

It cannot be that they who 'strict decorum's' law fulfil,
E'en in forgetful mood, should utter words of ill

Explanation
Those who study propriety of conduct will not speak evil, even forgetfully.

Tamil Transliteration
Ozhukka Mutaiyavarkku Ollaave Theeya
Vazhukkiyum Vaayaar Solal.