BG 2.47: You have a right to perform your prescribed duties, but you are not entitled to the fruits of your actions. Never consider yourself to be the cause of the results of your activities, nor be attached to inaction.
This is an extremely popular verse of the Bhagavad Gita, so much so that even most school children in India are familiar with it. It offers deep insight into the proper spirit of work and is often quoted whenever the topic of karm yog is discussed. The verse gives four instructions regarding the science of work: 1) Do your duty, but do not concern yourself with the results. 2) The fruits of your actions are not for your enjoyment. 3) Even while working, give up the pride of doership. 4) Do not be attached to inaction.
Do your duty, but do not concern yourself with the results. We have the right to do our duty, but the results are not dependent only upon our efforts. A number of factors come into play in determining the results—our efforts, destiny (our past karmas), the will of God, the efforts of others, the cumulative karmas of the people involved, the place and situation (a matter of luck), etc. Now if we become anxious for results, we will experience anxiety whenever they are not according to our expectations. So Shree Krishna advises Arjun to give up concern for the results and instead focus solely on doing a good job. The fact is that when we are unconcerned about the results, we are able to focus entirely on our efforts, and the result is even better than before.