Bg 3.9

yajnarthat karmano ’nyatra
loko ’yam karma-bandhanah
tad-artham karma kaunteya
mukta-sangah samacara

Word for word: 
yajna-arthat — done only for the sake of Yajna, or Visnu; karmanah — than work; anyatra — otherwise; lokah — world; ayam — this; karma-bandhanah — bondage by work; tat — of Him; artham — for the sake; karma — work; kaunteya — O son of Kunti; mukta-sangah — liberated from association; samacara — do perfectly.

Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Work done as a sacrifice for Visnu has to be performed; otherwise work causes bondage in this material world. Therefore, O son of Kunti, perform your prescribed duties for His satisfaction, and in that way you will always remain free from bondage.

Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Since one has to work even for the simple maintenance of the body, the prescribed duties for a particular social position and quality are so made that that purpose can be fulfilled. Yajna means Lord Visnu, or sacrificial performances. All sacrificial performances also are meant for the satisfaction of Lord Visnu. The Vedas enjoin: yajno vai visnuh. In other words, the same purpose is served whether one performs prescribed yajnas or directly serves Lord Visnu. Krishna consciousness is therefore performance of yajna as it is prescribed in this verse. The varnasrama institution also aims at satisfying Lord Visnu. Varnasramacaravata purusena parah puman/ visnur aradhyate (Visnu Purana 3.8.8).

Therefore one has to work for the satisfaction of Visnu. Any other work done in this material world will be a cause of bondage, for both good and evil work have their reactions, and any reaction binds the performer. Therefore, one has to work in Krishna consciousness to satisfy Krishna (or Visnu); and while performing such activities one is in a liberated stage. This is the great art of doing work, and in the beginning this process requires very expert guidance. One should therefore act very diligently, under the expert guidance of a devotee of Lord Krishna, or under the direct instruction of Lord Krishna Himself (under whom Arjuna had the oportunity to work). Nothing should be performed for sense gratification, but everything should be done for the satisfaction of Krishna. This practice will not only save one from the reaction of work, but also gradually elevate one to transcendental loving service of the Lord, which alone can raise one to the kingdom of God.