Exodus 7:9-10
7:9 “When Pharaoh says to you, ‘Do a miracle,’ and you say to Aaron, ‘Take your staff and throw it down before Pharaoh,’ it will become a snake.”
7:10 When Moses and Aaron went to Pharaoh, they did so, just as the Lord had commanded them – Aaron threw down his staff before Pharaoh and his servants and it became a snake.