9. When Pharaoh shall speak unto you, &c.--The king would naturally demand some evidence of their having been sent from God; and as he would expect the ministers of his own gods to do the same works, the contest, in the nature of the case, would be one of miracles. Notice has already been taken of the rod of Moses (Ex 4:2), but rods were carried also by all nobles and official persons in the court of Pharaoh. It was an Egyptian custom, and the rods were symbols of authority or rank. Hence God commanded His servants to use a rod.
JFB.
Picture Study Bible