Kural - 590

Reward not trusty spy in others' sight,
Or all the mystery will come to light

Explanation
Let not a king publicly confer on a spy any marks of his favour; if he does, he will divulge his own secret.

Tamil Transliteration
Sirappariya Otrinkan Seyyarka Seyyin
Purappatuththaan Aakum Marai.