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 (c) 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].

(c) Because these three signs should be sufficient witnesses to prove that Moses should deliver God's people.