The formula is that we have to become detached from the result of the work and must be situated in full knowledge, in full knowledge. Unless we are situated in full knowledge, it is not possible to be detached from the work which you are doing. And that detachment and that knowledge, to be situated in full knowledge, is possible when we perform yajna or sacrifice.
Lecture on Bhagavad-gita 4.20-24, New York, August 9, 1966