Kural - 661

What men call 'power in action' know for 'power of mind'
Externe to man all other aids you find

Explanation
Firmness in action is (simply) one's firmness of mind; all other (abilities) are not of this nature.

Tamil Transliteration
Vinaiththitpam Enpadhu Oruvan Manaththitpam
Matraiya Ellaam Pira.