Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 135
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 135, with English translation by Maimonides (Rambam).
לשבות מעבודת הארץ, שנאמר: "בחריש ובקציר, תשבות" (שמות לד,כא).
To refrain from agricultural work [in the seventh year], as [Exodus 34:21] states: "From plowing and harvesting, you shall rest."