Exodus 4:8-9

8“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.

9And 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.”