Rashi on Exodus 9:10/רש"י על שמות ט׳:י׳

1באדם ובבהמה UPON MAN AND UPON CATTLE — If you ask, “Whence did they obtain these cattle”? Has it not already been stated, (v. 6) “And all the cattle of Egypt died”? I reply that this judgment was decreed only against the cattle that were in the fields, as it is said, (v. 3) “[The hand of the Lord is] upon thy cattle which is in the field”, and everyone who feared the Lord took his cattle in-doors. So it is taught in the Mechilta in the comment on (Exodus 14:7) “And he took six hundred chosen chariots.”