Exodus 9:10

7 And Pharaoh sent, and, behold, there was not one of the cattle of the Israelites dead. And the heart of Pharaoh was hardened, and he did not let the people go.

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.

9 And it shall become small dust in all the land of Egypt, and shall be a boil breaking forth with blains upon man, and upon beast, throughout all the land of Egypt.

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.

11 And the magicians could not stand before Moses because of the boils; for the boil was upon the magicians, and upon all the Egyptians.

12 And the LORD hardened the heart of Pharaoh, and he hearkened not unto them; as the LORD had spoken unto Moses.

13 And the LORD said unto Moses, Rise up early in the morning, and stand before Pharaoh, and say unto him, Thus saith the LORD God of the Hebrews, Let my people go, that they may serve me.