And Moshe said, “By this you will know that Hashem has sent me to do all these acts, that it is not from my own heart”(16:28).
The language used in this Pasuk is somewhat repetitive. What is the difference between knowing that “Hashem sent Moshe” and that “it is not from [his] own heart?” The Rambam in his commentary on Mishna Sanhedrin enumerates the thirteen principles of faith. The seventh of these principles states the belief that Moshe was a true Navi and that no other Navi ever has or ever will supersede him. The eighth principle asserts that the Torah of Moshe is the word of Hashem and is not falsified or altered.
Korach and his followers denied both of these principles as can be seen in 16:3; “It is too much for you! For the entire assembly are all holy and Hashem is in their midst; so why do you exalt yourselves over the congregation of Hashem?” In other words Korach is accusing Moshe and Aharon of exalting themselves over Bnai Yisrael, which clearly denies the eighth principle. Rashi relates earlier in 16:1 quoting the Midrash that Korach ridiculed Moshe by asking, “Does a garment of Tekhelet need a Tekhelet string? Does a house full of Sefarim need a Mezuza? These things were not commanded by Hashem rather, you made them up yourself!” Indeed, the Yerushalmi paraphrases Korach as saying “The Torah is not from Hashem, Moshe is not a prophet, nor is Aharon a [legitimate] Kohen Gadol!” This clearly shows Korach’s denial of the seventh and eighth principles.
Therefore, in order for Moshe to refute Korach’s accusations Moshe made two statements. He declared that in Korach’s punishment he would see that “Hashem sent me to do all these acts.” - I have been true prophet of Hashem in everything that I have done. The second statement is that “it is not from my own heart,” “it,” referring to the Torah, was not my own fabrication rather, Hashem wrote it.
There is a story mentioned in Bava Basra 74a about Rabbah bar bar Channah. One time he was traveling in the desert when an Arab nomad offered to show him the place where Korach and his followers were buried alive in the ground. Rabbah bent down to the ground and heard the confession of these men, clearly reflecting their two sins: denial of the authenticity of the Torah and the denial of Moshe’s prophecy.