Exodus 6:12

Behold, the children of Israel have not hearkened to me; they gave no heed to what I have said, how then shall Pharaoh hear me? — If the anguish of their spirit makes them deaf to that which would compose and comfort them, much more will his pride and insolence, make him deaf to that which will but exasperate him. Who am of uncircumcised lips — He was conscious to himself that he had not the gift of utterance.