Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 309
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 309, with English translation by Maimonides (Rambam).
שלא לעבוד ולזרוע בנחל איתן, שנאמר "אשר לא ייעבד בו, ולא ייזרע" (דברים כא,ד).
Not to till or sow the land [around] a powerful river [at which atonement was made for an unsolved murder], as [Deuteronomy 21:4] states: "...which must never be tilled and never be sown."