Jesus answered them, “This is the work of God, that you believe in him whom he has sent.”
John 6:29

The context makes it obvious that "this is the work of God" doesn't mean "this is a work God does", but something required by God. Yeshua wasn't saying that believing is a "work" in the sense of earning your salvation. He was redirecting the people's attention from following a list of do's and don't's to putting their faith in the Messiah.