“Moses and Aaron came to Pharaoh and they did so, as the Lord commanded; Aaron cast his staff before Pharaoh and before his servants and it became a serpent” (Exodus 7:10). “Moses and Aaron came to Pharaoh” – should not Aaron have entered first, as he was greater in years than him? Why Moses? It is because he was greater than him in Egypt, as it is stated: “The man, Moses, too, was very great [in the land of Egypt]” (Exodus 11:3).
Alternatively, because the Holy One blessed be He rendered Moses as god over Aaron, as it is stated: “And you will be to him as god” (Exodus 4:16), and it is appropriate for the master to enter first. “And they did so, as the Lord commanded” – in that they did not do so until Pharaoh requested a wonder, just as the Holy One blessed be He said. At that moment, “Aaron cast his staff.”