"You shall not waive a sickle to your neighbor's standing grain"Deuteronomy 23:26.

[Though a worker may eat from the produce on which he is working (see Positive Commandment 201), this is subject to two rules: 1) If he is working on produce that is still attached to the ground, he may only eat from it after he has finished his work with that produce (e.g., once he has filled a basket with grapes that he is harvesting, he may eat of the grapes in there). 2) If he is working with already harvested produce, he may eat of the produce as long as he hasn't completely finished his work on the produce.]

It is forbidden for a worker to eat of produce he is working on [if it doesn't meet the two criteria established above, and it is also forbidden for him to take a break in order to eat from the produce; rather he must eat as he works].