The Torah in Parashat Behar presents the laws of Shemita – the seventh, "sabbatical" year when farmers were forbidden from working their lands, and were required to allow all people to freely partake of their fields’ produce. The opening verse of the Parasha states, "G-d spoke to Moshe at Mount Sinai," emphasizing that this Misva was told to Moshe at Sinai.
The commentators raise the question of why this point needed to be mentioned. After all, weren’t all the Torah’s laws conveyed to Moshe at Sinai? Why specifically in reference to Shemita did the Torah need to inform us that the command was taught at Mount Sinai?
Rav Zalman Sorotzkin (1880-1966) suggests an answer based on the Midrash’s interpretation of the verse in Tehillim (103:20), "Barechu Hashem Malachav Giboreh Koah Oseh Debaro" – "Bless Hashem, His angels, those mighty in strength who obey His word." The Midrash explains that this verse refers to two different groups of people. The phrase "His angels" refers to Beneh Yisrael at the time of Matan Torah, when they declared, "Na’aseh Ve’nishma" – "We will do and we will hear." With this declaration, they committed themselves to a set of obligations and restrictions which they had not yet heard. They announced their allegiance to G-d’s laws before learning what this entails, a level of commitment which is more "angelic" than human. They rose to the level of "Malachav," of G-d’s angels, by committing themselves unconditionally to whatever G-d commands them.
The next phrase in the verse – "those mighty in strength who obey His word" – refers to farmers who obey the laws of Shemita. This Misva, more so than perhaps any other Misva in the Torah, requires courage. Imagine a retailer with a store on Fifth Ave. shutting down his business for an entire year, during which time he keeps the door open with a large sign announcing, "All merchandise free for the taking." Without exaggeration, this is precisely what the Torah demands of farmers in Eretz Yisrael during Shemita. They are to shut down their enterprise for a full year and allow all people to help themselves to their produce. These farmers are "Giboreh Ko’ah," the courageous heroes of the nation.
Rav Sorotzkin suggests that this is the connection between Shemita and Mount Sinai. These two occasions marked the highest level of commitment to G-d. Both at Matan Torah and during Shemita, Beneh Yisrael expressed their firmly-held belief that the highest value is observing G-d’s laws. This belief is what led them to make the blind commitment of "Na’aseh Ve’nishma," and what led them to leave their fields every seven years. The law of Shemita is thus rooted in Sinai, in the firm faith displayed by Beneh Yisrael when they received the Torah and announced their willingness to obey all the Misvot regardless of the sacrifices it entails.