Kural - 612

In action be thou, 'ware of act's defeat;
The world leaves those who work leave incomplete

Explanation
Take care not to give up exertion in the midst of a work; the world will abandon those who abandon their unfinished work.

Tamil Transliteration
Vinaikkan Vinaiketal Ompal Vinaikkurai
Theerndhaarin Theerndhandru Ulaku.