John Gill Commentary Exodus 7:17

"Thus saith Jehovah, In this thou shalt know that I am Jehovah: behold, I will smite with the rod that is in my hand upon the waters which are in the river, and they shall be turned to blood." — Exodus 7:17 (ASV)

Thus says the Lord, in this you shall know that I am the
Lord
By the following instance of his power and vengeance:

behold, I will smite with the rod that is in my hand ;
which though in the hand of Moses, (Exodus 7:18) yet he being his ambassador, and representing him, is said to be in the hand of the Lord; and with this he threatens to smite

upon the waters which are in the river ;
the river Nile, and the canals thereof:

and they shall be turned to blood ;
and if this river was their god, it would abundantly appear that the God of the Hebrews was Jehovah, and above all gods, and particularly above theirs.