Kural - 784

Nor for laughter only friendship all the pleasant day,
But for strokes of sharp reproving, when from right you stray

Explanation
Friendship is to be practised not for the purpose of laughing but for that of being beforehand in giving one another sharp rebukes in case of transgression.

Tamil Transliteration
Nakudhar Poruttandru Nattal Mikudhikkan
Mersenaru Itiththar Poruttu.