C. F. Keil and Franz Delitzsch on Exodus 9:1-2

Exo 9:1-2

The fifth plague consisted of a severe Murrain, which carried off the cattle (מקנה, the living property) of the Egyptians, that were in the field. To show how Pharaoh was accumulating guilt by his obstinate resistance, in the announcement of this plague the expression, “If thou refuse to let them go” (cf. Exo 8:2), is followed by the words, “and wilt hold them (the Israelites) still” (עוד still further, even after Jehovah has so emphatically declared His will).