Exodus 4:10-
...Then Moses said to the Lord, "O my Lord, I am not eloquent, neither before nor since you have spoken to your servant; but I am slow of speech and slow of tongue."
So the Lord said to him, who has made man's mouth? Or who makes the mute, the deaf, the seeing, or the blind? Have not I, the Lord?
But he said, O my Lord please send by the hand of whomever else you may send."
So the anger of the Lord kindled against Moses, and He said: "Is not Aaron the Levite your brother? I know that he can speak well... now you shall speak to him and put the words in his mouth and I will be with your mouth and with his mouth and I will teach you what you shall do.
So he shall be your spokesman to the people. And he himself shall be as a mouth for you, and you shall be to him as God..."
