Exodus 4:14-17

Then the anger of the LORD was kindled and burned against Moses; He said, “Is there not your brother, Aaron the Levite? I know that he speaks fluently. Also, he is coming out to meet you, and when he sees you, he will be overjoyed. You must speak to him and put the words in his mouth; I, even I, will be with your mouth and with his mouth, and I will teach you what you are to do. Moreover, he shall speak for you to the people; he will act as a mouthpiece for you, and you will be as God to him [telling him what I say to you]. You shall take in your hand this staff, with which you shall perform the signs [the miracles which prove I sent you].”