John Gill Commentary Exodus 9:19

"Now therefore send, hasten in thy cattle and all that thou hast in the field; [for] every man and beast that shall be found in the field, and shall not be brought home, the hail shall come down upon them, and they shall die." — Exodus 9:19 (ASV)

Send therefore now, and gather your cattle, and all that you
have in the field
The servants that were at work there: this is said to denote both the certainty of the plague, and the terribleness of it, that all, both men and beast, would perish by it, if care was not taken to get them home; and also to show the wonderful clemency and mercy of God to such rebellious, hardened, and undeserving creatures, as Pharaoh and his people were; in the midst of wrath and judgment God remembers mercy: for upon every man and beast which shall be found in the field, and
shall not be brought home ;
and there sheltered in houses, barns, and stables: the hail shall come down upon them, and they shall die ;
the hailstones that would fall would be so large and so heavy as to kill both men and beasts, like those which fell from heaven upon the Canaanites in the days of Joshua, which killed more than the sword did, (Joshua 10:11) .