Bava Metzia 88 - Ox Eats While Harvesting
By Torah law, one is obligated to tithe only the produce that he grows and consumes - "... you shall surely tithe ..." and "... you shall eat ...", but not produce that one sells. However, the Sages have established that a tithing obligation is fixed also when the produce is sold or when Sabbath arrives.
The obligation not to stop your workers from eating at the time of harvesting is similar to the prohibition of " You shall not muzzle an ox in its threshing ," which applies to already harvested produce. Clearly, all animals are included, so by mentioning "ox" the Torah teaches another law: just as a worker is allowed to eat unharvested produce, so too an ox.