Exodus 7:12-16

For they cast down every man his rod, and they became serpents: but Aaron’s rod swallowed up their rods.

And he hardened Pharaoh’s heart, that he hearkened not unto them; as the LORD had said.

And the LORD said unto Moses, Pharaoh’s heart is hardened, he refuseth to let the people go.

Get thee unto Pharaoh in the morning; lo, he goeth out unto the water; and thou shalt stand by the river’s brink against he come; and the rod which was turned to a serpent shalt thou take in thine hand.

And thou shalt say unto him, The LORD God of the Hebrews hath sent me unto thee, saying, Let my people go, that they may serve me in the wilderness: and, behold, hitherto thou wouldest not hear.