The opening of this Parsha states that Hashem spoke to Moshe on Har Sinai telling him to speak to Bnai Yisrael about letting the land rest during the Shemittah year (ויקרא כ"ה:א',ב'). Many Meforshim point out that the Torah specifically states that this commandment was given on Har Sinai, but then note that in fact, all of the commandments were given on Har Sinai. Why, they then ask, does the Torah pick this Mitzvah with which to emphasize the place where it was received?
One answer is that this Mitzvah requires a special type of Emunah, faith, in Hashem, perhaps more so than any other Mitzvah, because it involves relying completely on Hashem for one's sustenance. Where do we see an example of this type of faith? This is the type of faith that was instilled in those who witnessed Mattan Torah at Har Sinai. If we consider a farmer, who relies on his land for food and as a source of money to keep himself and his family alive, and who is told that for one year, he can't plant or tend the crops, we realize that this requires an extraordinary amount of faith in Hashem. To muster such extraordinary faith, one must look back to a time when he already had experienced such faith. That time was at Mattan Torah on Har Sinai.
Faith came relatively easy to those who were present at Mattan Torah. The mere sight of Hashem's presence on Har Sinai was sufficient "nourishment" for them, as the Torah says elsewhere, "and they saw Hashem and they ate and they drank" (שמות כ"ד:י"א). The Kli Yakar (שם) comments that just the sight of Hashem that they experienced was better than food or drink. We also see this from Moshe himself who went forty days and nights without eating or drinking, because he was sustained by simply being in the presence of Hashem. The Har Sinai experience instilled faith into everyone who was present. Someone who witnessed that sight or even heard about it would find it much easier to forgo his normal cycle of farming and planting for one year, because he understands what faith in Hashem is. Therefore, to make sure that this Mitzvah would be kept properly, it was clearly identified with Har Sinai, and that is why this Mitzvah, of all the Mitzvos in the Torah, was presented along with a reference to Har Sinai.