Kural - 791

To make an untried man your friend is ruin sure;
For friendship formed unbroken must endure

Explanation
As those who are of a friendly nature will not forsake (a friend) after once loving (him), there is no evil so great as contracting a friendship without due inquiry.

Tamil Transliteration
Naataadhu Nattalir Ketillai Nattapin
Veetillai Natpaal Pavarkku.