Exodus 4:9

And it shall come to pass, if they will not believe also these
two signs
Performed before their eyes; for these were done over again when Moses came into Egypt to the Israelites, and yet some of them might still remain unbelievers to his commission, and so to the voice of these signs, which loudly called for their faith: neither hearken unto thy voice;
affirming he came from God, and was sent to be the deliverer of them: that thou shalt take of the water of the river;
of the river Nile, when he should come into Egypt; wherefore Josephus ^F17 is mistaken when he intimates that this was done at the same time with the other signs; and was water he took near at hand and poured on the ground: but Philo ^F18 truly refers this to Egypt, where it was done, as it ought to be: and pour [it] upon the dry land, and the water which thou takest out of
the river shall become blood upon the dry land;
by which it would appear how easily the Lord could destroy the land of Egypt, and make it a barren land, whose fertility was owing to the overflow of the river Nile as a means; and this would be a specimen also of what he would do hereafter, in turning the waters of the river into blood, thereby avenging the blood of innocent babes drowned there by the Egyptians.

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FOOTNOTES:

F17 Antiqu. l. 2. c. 12. sect. 3.
F18 De Vita Mosis, l. 1. p. 614.