Īśāvāsyopaniṣat - 3

kurvanneveha karmāṇi jijīviṣecchataṃ samāḥ |

evaṃ tvayi nānyatho~sti na karma lipyate nare || 2 ||

śataṃ samāḥ = For a hundred years, 

kurvan eva iha karmāṇi = certainly perform actions here

jijīviṣet - intend to live.

evaṃ tvayi = Be disposed to this discipline

nānyathā asti = There is no other discipline

na karma lipyate nare = Action does not stick to this person

Intending to live for a hundred years, a person should certainly engage in actions in this world. A disposition to act in this manner should be cultivated. There is no other way. 

Those who claim that actions are not required for people desiring liberation are cheating themselves and others. There is no way that the human condition can be restrained from acting in any way, physical or mental. As a consequence, we see monks, claiming to have abandoned everything, engaging themselves actively in political or financial pursuits, or going around with a bloated ego. It is impossible to give up all kinds of mental and physical action. It is also irresponsible, even if one were able to give up completely. One must also do one's might to the society. We certainly cannot live in a world full of monks who have given up all action. 

The 'actions' mentioned here are actions that are consistent with dharma, and not actions like revenge-mongering or cheating. 

The actions must be performed with the sense of vairāgya and ārāyaṇa-pāratantrya taught in the previous hymn. Performing actions endowed with this sense is the discipline to be cultivated. Than this discipline, there is no other way to be liberated. 

Some people object that the very nature of action (karma) is to stick to the person performing it and delivering its fruits. In that case, a person constantly engaging in action would get attached to the fruits of all kinds of karma. Liberation would then be impossible. With this argument, some teachers require the abandonment of all action.

The hymn denies this saying that one who performs action with the right disposition is unaffected by karma. Action only sticks to people and binds them if they are performed with lust and desire. But, if a person is enlightened by the knowledge in the previous hymn, there is no scope for imagining ownership or desire towards objects. One's actions become a form of service to Nārāyaṇa. It is called service or kaiṅkarya because what is performed without desire of reward is service. 

Since everything including oneself belongs to Vāsudeva, it is not a favor. A favor is done by an independent person. Since the soul belongs to Vāsudeva and is not independent, it is not a favor done in disinterested manner. This is the meaning of service. Since everything belongs to Vāsudeva, the fruit of the action also belongs to Vāsudeva. But, Vāsudeva is untainted by action. He is unaffected because it all belongs to Him anyway. There is no mistake here. But, the individual soul mistakes oneself to be independent or mistakes what belongs to Vāsudeva as one's own belonging. It is due to this error that the actions of the individual soul bind it.

But, if the individual soul performs actions with the knowledge that everything belongs to Vāsudeva, then there is no mistake. Therefore, action does not bind.

In this manner, the Upaniṣat beautifully explains the science of karma or action.