Rambam Mishna Torah – Yesodei Ha Torah – Chapter 8
The Jews did not believe in Moses, our teacher, because of the wonders that he performed. Whenever anyone’s belief is based on wonders, [the commitment of] his heart has shortcomings, because it is possible to perform a wonder through magic or sorcery.
All the wonders performed by Moses in the desert were not intended to serve as proof [of the legitimacy] of his prophecy, but rather were performed for a purpose. It was necessary to drown the Egyptians, so he split the sea and sank them in it. We needed food, so he provided us with manna. We were thirsty, so he split the rock [providing us with water]. Korach’s band mutinied against him, so the earth swallowed them up. The same applies to the other wonders.
What is the source of our belief in him? The [revelation] at Mount Sinai. Our eyes saw, and not a stranger’s. Our ears heard, and not another’s. There was fire, thunder, and lightning. He entered the thick clouds; the Voice spoke to him and we heard, “Moses, Moses, go tell them the following:….”
Thus, [Deuteronomy 5:4] relates: “Face to face, God spoke to you,” and [Deuteronomy 5:3] states: “God did not make this covenant with our fathers, [but with us, who are all here alive today].”
How is it known that the [revelation] at Mount Sinai alone is proof of the truth of Moses’ prophecy that leaves no shortcoming? [Exodus 19:9] states: “Behold, I will come to you in a thick cloud, so that the people will hear Me speaking to you, [so that] they will believe in you forever.” It appears that before this happened, they did not believe in him with a faith that would last forever, but rather with a faith that allowed for suspicions and doubts.
Thus, those to whom [Moses] was sent witnessed [his appointment] as a prophet, and it was not necessary to perform another wonder for them. He and they were witnesses, like two witnesses who observed the same event together. Each one serves as a witness to his colleague that he is telling the truth, and neither has to bring any other proof to his collegue.
Similarly, all Israel were witnesses to [the appointment of] Moses, our teacher, at the [revelation] at Mount Sinai, and it was unnecessary for him to perform any further wonders for them.
This concept [is alluded to in the interchange between God and Moses at the revelation of the burning bush]. At the beginning of his prophecy, the Holy One, blessed be He, gave him the signs [and wonders] to perform in Egypt and told him [Exodus 3:18], “And they will listen to your voice.”
Moses, our teacher, knew that one who believes [in another person] because of signs has apprehension in his heart; he has doubts and suspicions. Therefore, he sought to be released from the mission, saying: “They will not believe me” [Exodus 4:1], until the Holy One, blessed be He, informed him that these wonders [were intended only as a temporary measure,] until they left Egypt. After they would leave, they would stand on this mountain and all doubts which they had about him would be removed.
[God told him:] Here, I will give you a sign so that they will know that I truly sent you from the outset, and thus, no doubts will remain in their hearts. This is what is meant by [Exodus 3:12]: “This will be your sign that I sent you: When you take the people out of Egypt, you will serve God on this mountain.”
Thus, we do not believe in any prophet who arises after Moses, our teacher, because of the wonder [he performs] alone, as if to say: If he performs a wonder we will listen to everything he says. Rather, [we believe him] because it is a mitzvah which we were commanded by Moses who said: If he performs a wonder, listen to him.
Just as we are commanded to render a [legal] judgment based on the testimony of two witnesses, even though we do not know if they are testifying truthfully or falsely, similarly, it is a mitzvah to listen to this prophet even though we do not know whether the wonder is true or performed by magic or sorcery.
Translation from Chabad.