Rashi asks a critical question on the very first posukim of our Parsha:
“Hashem spoke to Moshe on Har Sinai, saying: Speak to the B’nai Yisrael, and say to them: When you come to the land which I give you, then shall the land keep a Shabbat to Hashem.” (Parsha Behar, Vayikra Perek 25, posukim 1 & 2)
Rashi asks why the laws of Shemittoh are singled out as having been given at Sinai. Were not all of the Mitzvot said at Sinai?” He answers that just as all of the Mitzvot; their general rules and their specifics were taught at Sinai, so too were the general rules and specifics regarding Shemittoh taught at Sinai. Rashi then reasons that the posuk comes to teach us that every utterance said to Moshe, they were all from Sinai. (Rashi on Parsha Behar, Vayikra Perek 25, posuk 1)