If they will not believe even these two signs or listen to your voice, you shall take some water from the Nile and pour it on the dry ground, and the water that you shall take from the Nile will become blood on the dry ground.”
Moses objects that the people will not believe him, yet God had promised they would hearken. His doubt does not square with God's word; past rejection made him fear the same again, though he was happily mistaken. The signs God appointed would convince them where bare word could not.
AI summary
Commenting on Exodus 4:1-9
These signs would be performed before their eyes when Moses came to Egypt, yet some might still refuse to believe his commission or heed his voice. The third sign, water from the Nile turned to blood on dry ground, showed how easily the Lord could strip Egypt of fertility, since the river alone made that land prosperous, and previewed the judgment He would pour out for the blood of infants drowned by the Egyptians.
AI summary
take of the water of the river--Nile. Those miracles, two of which were wrought then, and the third to be performed on his arrival in Goshen, were at first designed to encourage him as satisfactory proofs of his divine mission, and to be repeated for the special confirmation of his embassy before the Israelites.