Kural - 998

Though men with all unfriendly acts and wrongs assail,
'Tis uttermost disgrace in 'courtesy' to fail

Explanation
It is wrong (for the wise) not to exhibit (good) qualities even towards those who bearing no friendship (for them) do only what is hateful.

Tamil Transliteration
Nanpaatraar Aaki Nayamila Seyvaarkkum
Panpaatraar Aadhal Katai.