Albert Barnes on John 6:29

This is the work of God. This is the thing that will be acceptable to God, or which you are to do in order to be saved. Jesus did not tell them they had nothing to do, or that they were to sit down and wait, but that there was a work to perform, and that was a duty that was imperative. It was to believe on the Messiah. This is the work which sinners are to do; and doing this they will be saved, for Christ is the end of the law for righteousness to every one that believeth, Rom 10:4.

(m) "This is the work of God" 1Jn 3:23