Exode 4:3

And He said, Cast it on the ground; and he cast it to the ground, and it became a serpent; and Mosheh fled from before it.

AND HE SAID: CAST IT ON THE GROUND. AND HE CAST IT ON THE GROUND. I do not understand why G-d performed the signs before Moses. Moses believed that it is the Holy One, blessed be He, Who speaks with him, and it would have been fitting for Him to say, “The staff that is in your hand you shall cast on the ground before them, and it shall become a serpent,” and the same also with respect to the second sign, [i.e., his hand becoming leprous], just as He said at the third sign, and thou shalt take of the water of the river, and pour it upon the dry land, etc. It is for this reason that the words of our Rabbis can be relied upon, namely, that the first sign, [i.e., the staff turning into a serpent], was a hint to Moses that he had slandered the Israelites [when he said that they would not believe him],. Thus it suggested to her that it was not because it may bring about her death that G-d forbade it, but out of a sense of jealousy. As Rashi puts it: “Every artisan detests his fellow-artisans.” and the second sign was for the purpose of punishing him. And this is the sense of the expression, and Moses fled from before it [the serpent]. He feared lest he would be punished and the serpent would bite him, since every person naturally avoids danger, even though Moses knew that if it was indeed G-d’s desire [to punish him], there was no one that could deliver him out of His hand.
Perhaps even though He informed Moses of the Great Name with which the world was created and everything came into existence, He wished to show him that with this Name signs and wonders would be done, changing the natural order of things, so that the matter would be firmly established in Moses’ heart and that he should in truth know that with the Great Name he will perform new things in the world. The first two signs were sufficient for Moses, and therefore the third miracle of the water [turning into blood] was not done here. Instead, G-d commanded him to do the third sign in the sight of the people.

וינס משה מפניו, for the staff had become a real snake and had assumed a threatening posture. This was totally different from the “snakes” produced by Pharaoh’s magicians that were unable to move. They only looked like the real thing, but were totally harmless. None of these sorcerers can produce something that is really alive, i.e. becomes part of nature. Our sages in Sanhedrin 67 commented on this phenomenon by saying: “not only can the sorcerers not produce small creatures, they cannot even produce big creatures such as a camel” This is why in the case of Moses, the Torah speaks of “the staff which had turned into a נחש, serpent,” whereas when the sorcerers’ work is described the Torah speaks of תנין. [at that point Aaron’s staff also turned into a תנין, not נחש, as G’d was not yet ready to demonstrate the superior magic Moses was capable of. Only when Aaron’s staff swallowed the “taninim” of the sorcerers was that point brought home to Pharaoh.]

ויהי לנחש, “it turned into a serpent.” Why did G–d show Moses this miracle by using a serpent rather than any other creature? It symbolised that just as the bite of a snake not only hurts but results in the death of the victim, so Pharaoh and his servants would not only “bite” the Hebrews but would by their treatment of them cause their deaths. Moses being asked to grasp the tail of that serpent and turn it back into his staff, was to symbolise that Pharaoh and his servants would become dried out as the wood of his staff. Any hand afflicted with the plague of tzoraat had automatically become ritually unclean, as we know from Leviticus 14,6. Moses placing his hand back within the folds of his tunic symbolically restores the entire Jewish people to ritual purity which it had lost through contact with the Egyptians. When the Torah reports in verse 7 that והנה שבה כבשרו, “and lo, his hand looked again like the rest of his flesh,“ this was a hint that the Jewish people’s state of ritual impurity would be reversed.