Exodus 7:9-10

“When Pharaoh speaks to you, saying, ‘Show a miracle for yourselves,’ then you shall say to Aaron, ‘Take your rod and cast it before Pharaoh, and let it become a serpent.’ ” So Moses and Aaron went in to Pharaoh, and they did so, just as the LORD commanded. And Aaron cast down his rod before Pharaoh and before his servants, and it became a serpent.