Ibn Ezra on Exodus 7:15/אבן עזרא על שמות ז׳:ט״ו

1GET THEE UNTO PHARAOH IN THE MORNING. It is the custom of the king of Egypt until this very day to go out in the months of Tamuz and Ab to see how high the water has risen. This is the time the river increases. God commanded Moses to go in the morning and stand before the river and perform the sign of the striking of the river in the presence of Pharaoh. He commanded Moses to take and then give to Aaron the rod which he had turned into a serpent before coming unto Pharaoh (Ex. 4:3) and to wave it over the waters of Egypt so that Pharaoh would see with his very eyes the river turn into blood at the moment that Aaron struck it.