Exodus 9:2-3

9:2 For if you refuse to release them and continue holding them, 9:3 then the hand of the Lord will surely bring a very terrible plague on your livestock in the field, on the horses, the donkeys, the camels, the herds, and the flocks.

Exodus 9:17-18

9:17 You are still exalting yourself against my people by not releasing them. 9:18 I am going to cause very severe hail to rain down about this time tomorrow, such hail as has never occurred in Egypt from the day it was founded until now.