Positive commandment · Mitzvat Aseh #134 of 613
To let the land lie fallow [in the seventh year]
להשמיט קרקע
The verse · Shemot 23:11
וְהַשְּׁבִיעִת תִּשְׁמְטֶנָּה וּנְטַשְׁתָּהּ וְאָכְלוּ אֶבְיֹנֵי עַמֶּךָ וְיִתְרָם תֹּאכַל חַיַּת הַשָּׂדֶה כֵּן־תַּעֲשֶׂה לְכַרְמְךָ לְזֵיתֶךָ
“but the seventh year thou shalt let it rest and lie fallow; that the poor of thy people may eat: and what they leave, the beasts of the field shall eat. In like manner thou shalt deal with thy vineyard, and with thy olive grove.”
The Rambam’s words
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."
להשמיט קרקע, שנאמר: "והשביעית תשמטנה ונטשתה" (שמות כג,יא).
Why · Sefer HaChinuch #84
Releasing (shmitat) of lands
“It is from the roots of this commandment to affix in our hearts and make a strong impression in our minds [about] the matter of the world having been created.”
Who & when
“And [it] is practiced by males and females in the Land of Israel only, at the time that [the people of] Israel is there - as it is stated about it (Leviticus 25:2), “When you come to the Land.””
Where in the Mishneh Torah
Hilchot Sabbatical Year and the Jubilee הלכות שמיטה ויובל