Exodus 4:8-9, 15-16

“Then it will be, if they do not believe you, nor heed the message of the first sign, that they may believe the message of the latter sign. And it shall be, if they do not believe even these two signs, or listen to your voice, that you shall take water from the river and pour it on the dry land. The water which you take from the river will become blood on the dry land.”

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.