Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 134
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 134, with English translation by Maimonides (Rambam).
להשמיט קרקע, שנאמר: "והשביעית תשמטנה ונטשתה" (שמות כג,יא).
To let the land lie fallow [in the seventh year], as [Exodus 23:11] states: "In the seventh year, you shall let it lie fallow and withdraw from it."