Exodus 4:15 in the King James Version
And thou shalt speak unto him, and put words in his mouth: and I will be with thy mouth, and with his mouth, and will teach you what ye shall do.
Context
14 The LORD’s anger burned against Moses, and he said, “What about Aaron, your brother, the Levite? I know that he can speak well. Also, behold, he is coming out to meet you. When he sees you, he will be glad in his heart.
15 You shall speak to him, and put the words in his mouth. I will be with your mouth, and with his mouth, and will teach you what you shall do.
16 He will be your spokesman to the people. It will happen that he will be to you a mouth, and you will be to him as God.