Exodus 7:9

When Pharaoh shall speak unto you, saying, Shew a miracle for you: then thou shalt say unto Aaron, Take thy rod, and cast it before Pharaoh, and it shall become a serpent.

When Pharaoh shall speak ... Since the king would naturally demand some evidence of their having been sent from God, Moses and Aaron are not only prepared to expect such an inquiry into the credentials of their commission, but instructed in what form the demonstration should be given.

Show a miracle for you, х mowpeet (H4159)] - a wonder, a great and splendid deed (Gesenius); also a sign or proof of a divine commission (Deuteronomy 13:2-3; 1 Kings 13:3; 1 Kings 13:5). The Septuagint [Dote heemin seemeion ee teras] gives us a sign, or miracle. Both words are used. Deuteronomy 6:22; Nehemiah 9:10; Jeremiah 32:10. х 'owt (H226), sign strictly refers to the proof of their delegated character, such as Pharaoh demanded, while mowpeet (H4159) applies to the prodigies which followed.]

And it shall become a serpent, х yªhiy (H1961) lªtaniyn (H8577)] - shall be changed into a huge snake-like animal. It is a general term used for a sea monster (Genesis 1:21; Job 7:12); for a serpent (Deuteronomy 32:33; Psalms 91:13); for a dragon (Jeremiah 51:34); for a crocodile (Ezekiel 29:3). Here it denotes a serpent, as is indicated by the use of the specific term х naachaash (H5175) (Exodus 7:15; also Exodus 4:3); Septuagint, drakoon (G1404)]. Notice has already been taken of the rod of Moses (Exodus 4:2); but rods were carried also by all nobles and official persons in the court of Pharaoh. It was an Egyptian custom, and the rods were symbols of authority or rank. Hence, God commanded his servants to use a rod, the emblematical use of which was so well known that the word was synonymous with power, commission. The rod was not only an emblem of power, but the immediate means of executing it. It is called Aaron's rod, because, though it was really the rod of Moses, Aaron bore it, and, on a nod or sign from the leader, accompanied every word by an appropriate wave of that staff.