Exodus 7:14-16

14And the LORD said to Moses, Pharaoh’s heart is hardened, he refuseth to let the people go. 15Go to Pharaoh in the morning; lo, he goeth out to the water; and thou shalt stand by the river’s brink to meet him; and the rod which was turned to a serpent shalt thou take in thy hand. 16And thou shalt say to him, The LORD God of the Hebrews hath sent me to thee, saying, Let my people go, that they may serve me in the wilderness: and, behold, until now thou wouldest not hear.