Kural - 129

In flesh by fire inflamed, nature may thoroughly heal the sore;
In soul by tongue inflamed, the ulcer healeth never more

Explanation
The wound which has been burnt in by fire may heal, but a wound burnt in by the tongue will never heal.

Tamil Transliteration
Theeyinaar Suttapun Ullaarum Aaraadhe
Naavinaar Sutta Vatu.