When בני ישראל come to הר סיני, Moshe is given instructions for them:
What’s odd about this speech is the demands of כה תאמר and אלה הדברים אשר תדבר. That’s not how the words of ה׳ to Moshe are usually presented; usually it’s וידבר ה׳ אל משה לאמר. The “לאמר” means say it to others:
Rashi, from the Mechilta, says that this time there was an extra emphasis on saying it right:
Why? We don’t assume Moshe made any of the other speeches up. Why does this one need special emphasis to say כה תאמר?
Rav Yaakov Kaminetsky says that this is the introduction to מעמד הר סיני and the giving of the Torah, and as such, it had to be clear that everything that Moshe said was ה׳'s words, not his own. Other prophets get a Divine message but say it in their own words. Moshe is different:
The Torah comes from ה׳, not from Moshe. Other prophets are believed because of the signs they present and Moshe started that way:
But here it can’t come from Moshe, not even a single word:
Rav Kaminetsky adds:
The problem is that this isn’t true; Moshe did innovate at מעמד הר סיני:
ה׳ says He will manifest on the third day; Moshe tells the people to prepare for three days and ה׳ will manifest on the fourth. The gemara explicitly says this was Moshe’s innovation:
The Ramban (and Tosphot similarly) is bothered by the wording of the gemara. If it’s a דרשה from the wording, it’s not “מדעתו”):
The Maharal explains that this is the nature of תורה שבעל פה; it is ”מדעתם“ of חז״ל, even when derived from the words of G-d.
And it’s not just in the narrative of Torah; there are halachot that Moshe innovated:
Note how radical this is: הסכים הקדוש ברוך הוא עמו. The implication is that this is not what ה׳ meant by those words. In the “divine” תורה שעל פה, כן תעשה לכל הערים הרחקת ממך applies to וקראת אליה לשלום as well. But Moshe interpreted the law as he did, and as we know, (דברים ל:יב) לא בשמים היא. And ה׳ answers (בבא מציעא נט,ב) נצחוני בני.
Rav Meinzberg (I don’t know who he is but it’s presented on the Machon Herzog site) says that this is precisely the point of having Moshe innovate at the time of מעמד הר סיני:
The תורה שבכתב is fixed and absolute, אלה הדברים: לא פחות ולא יותר. The תורה שבעל פה is given to human beings to understand and הסכים הקדוש ברוך הוא עמו.