कर्मण्येवाधिकारस्ते मा फलेषु कदाचन। मा कर्मफलहेतुर्भूर्मा ते सङ्गोऽस्त्वकर्मणि ॥

karmaṇy-evādhikāras te mā phaleṣhu kadāchana mā karma-phala-hetur bhūr mā te saṅgo ’stvakarmaṇi

Thy business is with the action only, never with its fruits; so let not the fruit of action be thy motive, nor be thou to inaction attached.

In plain words: Krishna says you have a right only to do your work, never to control its results. Don't work just for the reward, but don't stop working either.