Exodus 7:8-10

Then the LORD said to Moses and Aaron, “When Pharaoh says to you, ‘Prove yourselves by working a miracle,’ then you shall say to Aaron, ‘Take your staff and cast it down before Pharaoh, that it may become a serpent.’” So Moses and Aaron went to Pharaoh and did just as the LORD commanded. Aaron cast down his staff before Pharaoh and his servants, and it became a serpent.