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.