Charles Ellicott 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)

In this thou shalt know that I am the Lord. — See the comment on Exodus 7:5.

The rod that is in my hand, that is, “in the hand of my servant.” God is here represented as about to do that which was actually done by Aaron (Exodus 7:20). “Qui facit per alium, facit per se.”