Exodus 4:8-9, 15-16

“If they will not believe you or pay attention to the evidence of the first sign, they may believe the evidence of the second sign. But if they will not believe these two signs or pay attention to what you say, you are to take some water from the Nile and pour it on the dry ground; and the water which you take out of the river will turn into blood on the dry ground.”

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].