Charles Ellicott Commentary Exodus 7:9

"When Pharaoh shall speak unto you, saying, Show a wonder for you; then thou shalt say unto Aaron, Take thy rod, and cast it down before Pharaoh, that it become a serpent." — Exodus 7:9 (ASV)

Show a miracle for you. —Pharaoh had perhaps heard of the miracles performed by Aaron before the people of Israel (Exodus 4:30), and was curious to be an eyewitness of one, as was Herod Antipas (Luke 23:8). Or he may have thought that if Moses and Aaron “showed a miracle,” his own magicians would be able to show greater ones, and he would then dismiss the brothers as charlatans and impostors. He certainly did not intend to be influenced by any miracle they might show, or to accept it as evidence that their message to him was a command from God.

Your rod. —The rod is now called Aaron’s because Moses had entrusted him with it. (Compare Exodus 7:19, Exodus 8:5, and Exodus 8:16-17.)

A serpent. —Or, a snake. The word is not the same as that used in Exodus 4:3, but appears to be a synonym.