Exodus 9:8

And the Lord said unto Moses and unto Aaron, Take to you handfuls of ashes of the furnace, and let Moses sprinkle it toward the heaven in the sight of Pharaoh.

Cross-reference

Exodus 8:16 describes a similar plague initiation: dust turned into gnats, just like here ashes become boils.

Revelation 16:2 echoes this plague: the first bowl brings foul sores on those with the beast's mark.