Exodus 9:10

And they took ashes of the furnace, and stood before Pharaoh; and Moses sprinkled it up toward heaven; and it became a boil breaking forth with blains upon man, and upon beast.

Cross-references

Exodus 15:26 promises that obeying God prevents the diseases of Egypt — the boils are an example of such a disease sent as judgment.

Deuteronomy 28:27 refers to 'the boils of Egypt' as a curse, directly echoing the plague described here.