This is the finger of God — The power of God. The devil's agents, when God permitted them, could do great things; but when he laid an embargo upon them, they could do nothing. The magicians inability in this instance shewed whence they had their ability in the former instances, and that they had no power against Moses but what was given them from above. But Pharaoh's heart was hardened — By himself and the devil.
Related Commentaries of Exodus 8:19
Exodus 8:19
19 Then the magicians said unto Pharaoh, This is the finger of God: and Pharaoh's heart was hardened, and he hearkened not unto them; as the LORD had said.