Exodus 16:20

But (some of them) hearkened not to Mosheh: Dathan and Abiram, men of wickedness, did reserve of it till the morning; but it produced worms and putrefied; and Mosheh was angry with them.

AND IT BRED WORMS, AND IT STANK. “This is a verse that is to be transposed, because [the manna that was left over until the morning] must have first stunk and afterwards become wormy, just as it is said, and it did not stink, neither was there any worm therein, this being the way of all things that become wormy.” Thus Rashi’s language. Now if the manna had become wormy in a natural way, as is the way of all things that become wormy, Rashi’s statement would be correct. But since the manna became wormy in a miraculous way, it is possible that it bred worms first, and there is thus no need for inverting] the verse. Moreover, the verse which states [concerning the manna which they retained for Sabbath morning], and it did not stink, neither was there any worm therein, proves that it was so, [i.e., that the manna left over by Dathan and Abiram bred worms first and then stank]. Had it not become wormy until it first stank, when Scripture says [concerning the manna left for the Sabbath], and it did not stink, it would thereby have already assured us that there was no worm therein. Why then should it repeat afterwards, neither was there any worm therein? If however, as the plain meaning of Scripture indicates, the manna that was left over until the morning by Dathan and Abiram became wormy first [in a supernatural way], it became necessary for Scripture to state that this manna [that was left over for the Sabbath] did not stink, nor did any worm come therein at all. Even things which become wormy in a natural way do not give off a stench unless they are warm and moist, but dry things only become wormy and do not give off an odor at all, just as wormy wood or fruits that become wormy when still growing or [immediately] afterwards. Thus Scripture relates that this manna [which Dathan and Abiram left over on a weekday, also stank [in the morning] by a miracle. And in Eileh Shemoth Rabbah we find that the Rabbis have said: “Is there anything that first becomes wormy and then gives off a stench? It is only that the Holy One, blessed be He, wanting to expose the deeds [of Dathan and Abiram] to the people, therefore caused [the left-over manna] to give off no stench at night lest they throw it out. Instead, during the entire night, it formed rows upon rows of worms, and at once Moses was wroth with them.”

ויקצף עליהם משה, the reason that Moses was angry was that the leftover manna was not the result of it being more than the people could eat, but because it represented an attempt by the people to find out if what they had been warned of would really happen.