“Hashem spoke to Moshe on Mount Sinai, saying: Speak to the Children of Israel and say to them, ‘When you come to the land which I give to you, the land shall observe a Shabbos rest for Hashem.’” 25:1,2
- What is the connection between the Sabbatical year and Mount Sinai? Were not all the mitzvos said at Sinai? Rather, this comes to teach us that just as the general rules, details, and specifications of the Sabbatical year were all said at Sinai, so too, the general rules, details, and specifications of all the Mitzvos were said at Sinai… - Rashi, Torah Kohanim
- Perhaps the reason that the mitzvah of Shmittah/ The Sabbatical Year was selected as the model for all the Mitzvos is because it is similar in nature and rationale to the mitzvah of Shabbos, which was heard at Sinai from the mouth of Hashem, and is an extension of that mitzvah. Based on this similarity, one might have mistakenly thought that it wasn’t necessary to have a full discussion of Shmittah at Sinai. Therefore, the Torah highlights the fact that Shmittah too was taught to the people in all-inclusive detail at Mt. Sinai.- Rabbeinu Bechaya
In addition to the numerous elucidations on this idea, I would add that perhaps the reason this mitzvah was chosen as an example of the extensive detail with which each mitzvah was presented on Mt. Sinai, is because of its great difficulty in observing. The laws of Shmittah demand great self-sacrifice and an abundance of faith in order to properly comply with. Perhaps, one may claim that on Sinai, only the “easy” Mitzvos were on display, thus misleading the people into believing that Judaism wasn’t that demanding after all. To disabuse us of this notion, the Torah goes out of its way to specify that even the laws of the Shmittah, with all their attendant detail and difficulty, were expounded upon at Sinai, and nevertheless, the Jewish people still insisted on entering the Covenant with Hashem.