Rashi, Leviticus 25:1

on Mount Sinai - What [special relevance] does the subject of Shemittah [the “release” of fields in the seventh year] have with Mount Sinai? Were not all the commandments stated from Sinai? However, [this teaches us that] just as with Shemittah, its general principles and its finer details were all stated from Sinai, likewise, all of them were stated-their general principles [together with] their finer details-from Sinai. This is what is taught in Torath Kohanim (25:1). [And why is Shemittah used as the example to prove this rule,especially since the very fine details are not even specified here (Sefer Hazikkaron)?] It appears to me that its explanation is as follows: [At the plains of Moab, Moses reiterated the majority of the laws of the Torah to the Israelites before their entry into the land of Israel, this reiteration comprising most of the Book of Deuteronomy. Now,] since we do not find the laws of Shemittah [“release”] of land reiterated on the plains of Moab in Deuteronomy, we learn that its general principles, finer details, and explanations were all stated at Sinai. Scripture states this [phrase] here to teach us that [just as in the case of Shemittah,] every statement [i.e., every commandment] that was conveyed to Moses came from Sinai, [including] their general principles and finer details [and that the commandments delineated in Deuteronomy were merely] repeated and reviewed on the plains of Moab [not originally given there]. [translation by Judaica Press]