Midrasch zu Schemot 9:14

And the Lord said unto Moses: “Rise up early in the morning, and stand before Pharaoh; lo, he cometh from the water” (Exod. 8:16). Why did Pharaoh go to the waters early in the morning? Because the wicked one boasted that since he was a god, he had no need to go to the water to relieve himself. Therefore he went out early in the morning so that no one would see him performing a demeaning act. That is why the Holy One, blessed be He, said to Moses: Rise up early in the morning, when he must (go out), and say unto him: Thus saith the Lord, the God of the Hebrews … for I will this time send all My plagues upon thy person, etc. (Exod. 9:14). Ordinarily, when a man desires to harm his enemy, he does so unexpectedly lest he be forewarned, but the Holy One, blessed be He, warned the wicked Pharaoh before each plague, as it is said: Behold, I will cause it to rain (ibid., v. 18), Behold, tomorrow will I bring locusts into thy border (ibid. 10:5). And He did so in each instance.