Exodus 4:9

And it shall come to pass, if they will not believe also these two signs, neither hearken unto thy voice, that thou shalt take of the water of the river, and pour it upon the dry land: and the water which thou takest out of the river shall become blood upon the dry land.

Take ... water ... out of the river - Nile. Those miracles, two of which were performed then, and the third was to be performed on his arrival in Goshen, were at first designed to encourage himself as satisfactory proofs of his divine mission, and to be repeated for the special confirmation of his embassy before the Israelites. The impression they were calculated to make upon the beholders is well described by Dr. Pearson ('On the Creed,' Art. 1):-`They who saw in Moses' hand God's omnipotency, could not suspect in his tongue God's veracity, insomuch that when Aaron became to Moses instead of a mouth, and Moses to Aaron instead of God, Aaron spake all the words which the Lord had spoken unto Moses, and did the signs in the sight of the people and the people believed.'