Shavuot: Torah Ohr: Studying at Sinai
The most important aspect of the receiving of the Torah is that which is written in the 10 statements: “and God spoke all of these words, saying (Exodus 20:1).” At first glance the word “saying” is incomprehensible. It is not comparable to any other leimor in the entire Bible, which is ordinarily interpreted as “saying” to another. Here it is impossible to explain it in this manner, for all Israel heard when God spoke to them face to face, those then present and those not present.
The correct interpretation of Leimor in this verse is: to utter and speak all of the words of the Torah that were already stated to Moses in the entire Bible; for Mishna, Halachot, and Aggadot, all of it was related to Moses at Sinai. Thus, although the Talmud cites the names of individual Tannaim and Amoraim as articulating a specific halachah, its meaning is that this word of God is the Law spoken to Moses at Sinai which emerged from the mouth of that rabbi.
Israel was granted this power that the law emerging from their mouth is actually the word of God spoken to Moses at Sinai, given at the time of the receiving of the Torah by Moses. (Torah Ohr, Yitro)