The 309th prohibition is that we are forbidden from sowing or working [the land around1] the 'strong river' where the calf is decapitated.2

The source of this prohibition is G‑d's statement,3 "[The elders of that city shall bring the calf to a swiftly flowing stream, the land around which] must never be worked or sown."

The punishment for this prohibition is lashes. In the enumeration of transgressions which are punished by lashes in tractate Makkos, our Sages said, "But you omitted one who sows the land around the 'strong river' — which is prohibited by the verse, 'It must never be worked or sown!' " This shows that there is one prohibition4 and that it is punishable by lashes.

The details of this mitzvah are explained in the end of tractate Sotah.