Bhagavad Gita · Chapter 3: The Path of Action
Bhagavad Gita 3.20
कर्मणैव हि संसिद्धिमास्थिता जनकादयः ।
लोकसंग्रहमेवापि संपश्यन्कर्तुमर्हसि ॥
karmaṇaiva hi saṃsiddhimāsthitā janakādayaḥ | lokasaṃgrahamevāpi saṃpaśyankartumarhasi ||
It was through action alone that Janaka and others attained perfection. You too should act, keeping in view the good of the world.
Modern context
Krishna gives an example and a new reason. The example is King Janaka, who reached perfection while ruling a kingdom. The reason is lokasangraha, holding the world together: the welfare and cohesion of society. Even someone who needs nothing for himself has cause to work, because others depend on it.