Kural - 185

The slanderous meanness that an absent friend defames,
'This man in words owns virtue, not in heart,' proclaims

Explanation
The emptiness of that man's mind who (merely) praises virtue will be seen from the meanness of reviling another behind his back.

Tamil Transliteration
Aranjollum Nenjaththaan Anmai Puranjollum
Punmaiyaar Kaanap Patum.