Deuteronomy 24:19 "If you have forgotten a sheaf in the field, you shall not go back to fetch it, it shall be for the stranger, for the fatherless and for the widow"

Even after the reaping in the field has been concluded, there is still another opportunity for the farmer to supply the poor man with his needs.

The produce, which by now has been piled in bundles, must be collected.

If a bundle is forgotten, the farmer may not go back to collect it. It must be left in the field for the needy people (see Negative Mitzvah 214).