Sforno on Exodus 4:15/ספורנו על שמות ד׳:ט״ו

1ושמת את הדברים בפיו ואנכי אהיה עם פיך ועם פיהו, even though I could put all these words in his mouth this is not sufficient; it is necessary that I personally, assist both you and him to make sure that Pharaoh will absorb the words with his heart. Otherwise, he might deal with you from a position of autocratic arrogance and simply expel you from his presence.