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.
The verse belongs to God's commission of Moses when the time for Israel's deliverance from Egypt had come. It gives a further, conditional sign: if the people do not believe the first two signs nor heed Moses' voice, water taken from the river and poured on the dry land will become blood upon the dry land. The signs were given to assure Moses when he feared that Israel would not believe him nor listen to him. They confirmed that Moses' message came from God.
By these signs and wonderful works God would convince Pharaoh and his people that One greater than Moses was manifesting power before them. By these signs God would cause Pharaoh and the Egyptians to fear, so that they would not dare to harm Moses. God purposed to accomplish the deliverance in a way that would pour contempt on human pride, sending the deliverer as a humble shepherd with only a rod in his hand and making that rod the symbol of divine power. As with Hezekiah's requested sign that God would heal him, Moses' signs show that a sign from heaven can confirm a divine promise.