The 219th prohibition is that we are forbidden to prevent an animal from eating from the produce it is working with as it works. If, for example, it is treading grain or carrying straw on its back, one may not prevent it from eating from the grain or straw.

The source of this prohibition is G‑d's statement,1 "Do not muzzle an ox when it is treading grain."

It is explained2 that the verse [says "ox" rather than "animal" because it] refers to the most common case, but one may not muzzle an ox nor any other animal. So too, one may not prevent it from eating the food as it works whether it is treading or doing another type of work. One who does so is punished by lashes, even if done only verbally.3

The details of this mitzvah are explained in the 7th chapter of tractate Bava Metzia.4