Kural - 510

Trust where you have not tried, doubt of a friend to feel,
Once trusted, wounds inflict that nought can heal

Explanation
To make choice of one who has not been examined, and to entertain doubts respecting one who has been chosen, will produce irremediable sorrow.

Tamil Transliteration
Theraan Thelivum Thelindhaankan Aiyuravum
Theeraa Itumpai Tharum.