Exodus 7:9-10

When Pharaoh shall speak unto you, saying, Shew a wonder for you: then thou shalt say unto Aaron, Take thy rod, and cast it down before Pharaoh, that it become a serpent. And Moses and Aaron went in unto Pharaoh, and they did so, as the LORD had commanded: and Aaron cast down his rod before Pharaoh and before his servants, and it became a serpent.