Pharaoh made himself into a god, as it is said (Ezekiel 29:9): "Because he said, 'The Nile is mine, and I made it.'" He said: I am the one who created myself. Immediately the Holy One, blessed be He, made known to him that he was flesh and blood. The Holy One, blessed be He, said: Because he has made himself a god, make known to him that he is flesh and blood. Behold, he goes out on his way and for his need in the morning; seize him and make known to him that he is flesh and blood. What did Moses do? Immediately, when the Holy One, blessed be He, said to him, "Rise early to Pharaoh," Moses seized him. He said: Leave me, that I may relieve my need, and afterward I will speak with you. Moses said: Is there a god who relieves his need? Therefore the Holy One, blessed be He, said to him (Exodus 9:13): "Rise early in the morning," and make known to him that he is flesh and blood.
Why Moses Caught Pharaoh at the Nile Each Morning
Midrash Tanchuma Buber, Vaera 16:4
פרעה עשה את עצמו אלוה, שנאמר יען אמר (יאור לי) [לי יאורי] ואני (עשיתיני) [עשיתי] (יחזקאל כט ט), אמר אני הוא שבראתי את עצמי, מיד הודיעו שהוא בשר ודם, אמר הקב"ה בשביל שעשה את עצמו אלוה, הודיעו שהוא בשר ודם, הרי הוא יוצא לדרכו ולצורכו בבוקר, אחוז בו והודיעו שהוא בשר ודם, מה עשה משה מיד כשא"ל הקב"ה השכם אל פרעה, אחז בו משה, אמר הנח לי שאעשה צרכי ואחר כך אני מדבר עמך, אמר משה יש אלוה שהוא עושה צרכיו, לפיכך אמר לו הקב"ה השכם בבוקר והודיעו כי הוא בשר ודם.