Leviticus 25:1

At Mount Sinai. Why is the matter of shmittah (Sabbatical year) found at Mount Sinai? Weren't all of the commandments spoken from Sinai? How is this the case with shmittah? It is spoken of in generalities and specifics and its details from Sinai. Even so all of them (the commandments) are spoken of in their generalities and details from Sinai, thus it is taught in Torat Kohamin (Leviticus). And it appears to me that that thus is its explanation: since we do not find the Sabbatical year for the soil that was taught on the plains of Moab in the book of Deuteronomy. That is to teach us that all of its details and its specifics were spoken at Sinai. And Scripture comes and teaches here on every speech that was attributed to Moses, that from Sinai was it given, all of their generalities and their details, and that they were repeated again on the plains of Moab.

AND the Lord spake with Mosheh in the mountain of Sinai, saying:

The Torah does not always follow chronological order. This passage was given before the parasha of Vayyiqra and the parashiyyot that follow it, since it was said on Mount Sinai . It concludes the covenant [26:45] that was mentioned in the parasha of Mishpaṭim [Exodus 24:7]. It appears here, out of order, to connect the laws of incest with the conditions under which the land was given, because incest was one of the things for which the land would vomit them out [18:28]. The sabbatical years are the first condition listed (they are also discussed in the parasha of Beḥuqqotai [26:34 ff]).

AND THE ETERNAL SPOKE UNTO MOSES IN MOUNT SINAI. “What has the subject of the Sabbatical year [stated here in this section] to do with that of Mount Sinai? Were not all the commandments declared on Sinai? [It is to teach us that] just as [this law of] the Sabbatical year was ordained on Sinai with its general rules, its specific prescripts and its minute details, so also were all [the commandments] ordained on Sinai with their general rules, and their specific prescripts and their minute details. Thus it is taught [here] in the Torath Kohanin. It appears to me (Numbers 22:1), Moses, in the last days of his life, restated the commandments of G-d, his words being contained in the Book of Deuteronomy. However, since not all the commandments were restated there by Moses, our verse serves as a basis for the interpretation of the Torath Kohanim, as will be explained by Rashi. Ramban will differ with this whole approach, since the text of the Torath Kohanim makes no reference at all to Moses’ restatement of the laws in the plains of Moab. See at end of Note 10. that the following is the interpretation thereof: Since we do not find the law of the resting of the soil [in the Sabbatical year] repeated in the plains of Moab [when Moses restated the law, as contained in the Book of Deuteronomy], we must deduce that all the general rules and specific prescripts [of the Sabbatical year, including those of the resting of the soil] were declared on Sinai. [Why then was it necessary for Scripture to state here in Mount Sinai, since it is self-understood that this entire law was promulgated on Sinai, since the law of the resting of the soil is not mentioned in the Book of Deuteronomy? We must say that the reason] Scripture mentioned [the phrase in Mount Sinai] here is in order to teach that every communication [i.e., commandment] that was spoken to Moses on Sinai, was given at Sinai together with its general rules and minute details, and they were [merely] repeated and taught again in the plains of Moab.” Thus far is the language of Rashi.
But this does not appear to me to be correct at all. For there are many commandments like the Sabbatical year which were not repeated in the plains of Moab, and [all] we know about their general rules and specific prescripts is that they were promulgated either on Sinai or in the Tent of Meeting [but not specifically on Sinai, as Rashi said]! [and are recorded in other parts of the Torah], were declared at some …

בהר סיני, before the Tabernacle had been erected.

וידבר ה' אל משה בהר סיני, it is noteworthy that nowhere else when the Torah announces new legislation does it also mention where this legislation was originally issued. The only time such locations are mentioned is when a new element of that commandment is added. We have heard about the principle of the sh’mittah legislation in Exodus 23,11 where the general outline was presented, the Torah writing that the produce of the land during the seventh year should be left untouched by the farmer and is intended for the poor of the nation.
Now the Torah reverts to present this legislation in greater detail. This is the reason why we are told where these details were first spelled out. What we read here is only a prototype. The principle does not apply only to the sh’mittah legislation, but wherever some legislation had been mentioned in a general manner only, when the details were filled in this took place at Mount Sinai, just as it did in the case of the sh’mittah legislation. (Torat Kohanim 1,1)
The reason that Moses mentioned this particular legislation at this point is because at this time he was still under the impression that the Israelites would proceed in short order to the land of Canaan, the sin of the spies which set them back by forty years not having occurred yet. Even as late as Numbers 10,29 Moses still spoke to the people of the impending journey to the Holy Land. Speaking about the soil of the holy Land “resting,” is the subject in Leviticus 26,34 as well as in Chronicles II 36,21. This shows that the main focus of the sh’mittah legislation concerns the “Sabbatical” experienced by the farmland of the nation.

בהר סיני, “at Mount Sinai;” all the portions preceding this one from the beginning of the Book of Leviticus had been revealed to Moses from the Tabernacle, (G–d’s voice emanating from the top of the Hoy Ark) and they all concerned laws of importance for the service in the Tabernacle in one way or another. The two portions commencing here had been revealed to Moses at Mount Sinai, as they neither concerned the Tabernacle nor the priests. This is the reason why the Torah here added the words: “at Mount Sinai.” Nonetheless, they were included in the Book of Leviticus, seeing that they deal with the laws of sh’mittah and Yovel as well as the laws of Erchin and Charamim which are administered by the priests. In verse 9 of our chapter the Torah decrees the blowing of the shofar on the tenth of Tishrey when the Jubilee year officially begins. This blowing of the shofar was performed by one of the priests. The fact that this blowing of the shofar when required was performed by the priests is documented in Numbers chapter 10, verse 8. In order that we should not think that these laws were issued from the Tabernacle, the Torah added the words: בהר סיני, “at Mount Sinai.”