Tirukkural-On Virtue-Avoiding Slander-Kural 184

Whatever evil is said in a man's presence is not so reprehensible,As that which is said behind his back.

Apropos of this, Herodotus said, 'The slanderer inflicts wrong by calumniating the absent'. True, some of the harshest things said to a man's face are preferable to the sneaky things that are whispered behind one's back. That is why valluvar drives home the point here that the worst evil spoken in a man's presence is not half so bad as speaking ill of him when he is not present.

Kalithogai makes mention of the idea in the following line.
Kannindru koorudha laatraanavanaayin