Take to you handfuls of ashes, to mind them of their cruel usage of the Israelites in their furnace, of which see Deuteronomy 4:20 Jeremiah 11:4. Both were to take them up, but Moses only to sprinkle them, as at other times Aaron only did the work, to show that they were but instruments, which God could use as he pleased, and God was the principal author of it.
Related Commentaries of Exodus 9:8
Exodus 9:8
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.