Deuteronomy 23:25 "When you come into your neighbor's vineyard, then you may eat your fill of grapes at your desire; but you shall not put any in your container"

As mentioned, a hired worker may eat from the fruit or grain that he is harvesting. However, there are limits to how much he is allowed to eat.

It would be cruel to stop a worker from eating entirely, but this Negative Mitzvah forbids a worker from taking more than a fair share.

A hired worker in a vineyard may only eat enough grapes to satisfy his hunger. He is not allowed to gather grapes for himself and pack them away to take home or eat later.