कार्यमित्येव यत्कर्म नियतं क्रियतेऽर्जुन। सङ्गं त्यक्त्वा फलं चैव स त्यागः सात्त्विको मतः ॥

kāryam ity eva yat karma niyataṁ kriyate ‘rjuna saṅgaṁ tyaktvā phalaṁ chaiva sa tyāgaḥ sāttviko mataḥ

He who performeth a prescribed action, saying, "it ought to be done," O Arjuna, relinquishing attachment and also fruit, that relinquishment is regarded as pure.

In plain words: Krishna says when you do your duty simply because it should be done, without clinging and without wanting rewards, that is the pure kind of letting go.