פירוש על שמות 9:22

על השמים signifies towards the heavens (i. e. על is the equivalent of אל). A Midrashic comment is that the Holy One, blessed be He, raised Moses high above the heavens see Midrash Tanchuma, Vaera 15).

Over the heavens. I.e., at first Moshe had control only over matters on earth. Now Hashem lifted Moshe up and gave him control over matters in heaven, and Moshe brought a plague from heaven.