Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 136
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 136, with English translation by Maimonides (Rambam).
לקדש שנת יובל בשביתה כשמיטה, שנאמר: "וקידשתם, את שנת החמישים" (ויקרא כה,י).
To sanctify the Jubilee year by refraining [from agricultural work], as is done in the shemitah, as [Leviticus 25:10] states: "And you shall sanctify the fiftieth year...."