Rashi on Exodus 7:15/רש"י על שמות ז׳:ט״ו

1הנה יצא המימה LO, HE GOETH OUT UNTO THE WATER to ease himself. For he claimed to be a god and asserted that because of his divine power he did not need to ease himself; and therefore he used to rise early and go to the Nile and there eased himself in secret (Midrash Tanchuma, Vaera 14; Exodus Rabbah 9:8).