Halakhah sa Exodo 3:12: Mishneh Torah, Foundations & Sefer HaChinukh

Consequently they to whom he was sent are the witnesses by whom the truth of his prophecy is established, wherefor he needed no other token to deliver to them, as they and he witnessed it together, and as when two witnesses see one thing together each one testifies for the other that he speaks the truth, and neither needs other testimony for the other. Thus, Moses our Master, all Israel are his witnesses since their presence at Mount Sinai, and there was no need for him to deliver to them any token. This is what the Holy One, blessed is He! spoke unto him, when he commenced to prophesy, at the time He endowed him with the tokens to be delivered in Egypt, saying to him: "And they shall hearken to thy voice" (Ex. 3.18). Moses our Master knew that whosoever believes on the testimony of tokens retains in his heart suspicions and continues to think and reflect; and he endeavored not to go, saying: "But, behold, they will not believe me" (Ibid. 4.1). Then it was that the Holy One, blessed is He! made known to him that these tokens were of no need save until they go forth out of Egypt, but after they go forth, when they will stand upon this Mount the suspicious thoughts which they might be thinking after thee will disappear, for I am herewith investing thee with a token which they will know in truth that it was I Who sent thee from the very start, so that there be left no suspicion in their heart. And it is this what the Verse says: "And this shall be the token unto thee, that I have sent thee: when thou hast brought forth the people out of Egypt, ye shall serve God upon this mountain" (Ibid. 3. 12). Consequently, it must be said that in every prophet who rises up after Moses our Master, we do not believe because of the token alone, so as to say: "If he deliver a token we will hearken to all of his prophesying"; but because of the commandment which Moses commanded in the Torah and said, if he do give a token, "Unto him ye shall hearken" (Deut. 18.15). Even as he commanded us to render decision in a matter on the testimony of two witnesses, although we do not know whether they have testified truth or falsehood, so it is mandatory to hearken to such prophet, notwithstanding our lack of knowledge whether the token is true or whether it was delivered by witchcraft and enchantment.

It is from the roots of the commandment from the angle of the simple understanding [that it is] since the entire essence of Israel is only the Torah, and because of the Torah were the heavens and earth created, and as it is stated (Jeremiah 33:25), “Were it not for my covenant day and night, etc.” And it is the essence and the reason that they were redeemed and left from Egypt — in order that they receive the Torah at Sinai, and fulfill it. And [it is] like God said to Moshe (Exodus 3:12), “And this will be the sign for you that I have sent you; when you take out the people from Egypt, you shall worship God on this mountain.” And the understanding of the verse is [that] your taking them out from Egypt is a sign for you that you shall worship God on this mountain — meaning that you shall receive the Torah, which is the great principle for which they were redeemed and it is their ultimate good. And it is a great matter for them, more than freedom from slavery. And hence God made a sign of their leaving Egypt for the receiving of the Torah; as we always make what is secondary into a sign for what is the essence.