Parashas Eikev · Passuk 9:10
And the ETERNAL gave me the two tablets of stone inscribed by the finger of God, with the exact words that the ETERNAL had addressed to you on the mountain out of the fire on the day of the Assembly.
Torah Temimah points to Megillah 19b.
TT lemma: ככל הדברים וגו'.
English: R’ Chiyya bar Abba said in the name of R’ Yochanan: What is the meaning of “and upon them according to all the words that Hashem spoke with you on the mountain”? It teaches that the Holy One showed Moshe the fine details of Torah, the fine details of the Scribes, and what the Scribes would later innovate; and what is this? The reading of the Megillah.
Custom (Sinai includes future derashot of the Soferim)
Rabbinic law and the methods of derash are not outside the Torah given at the mountain; Moshe was shown their foundation.