finger. Figure of speech Anthropopatheia (App-6). Note, the Egyptians say "God", not Jehovah.
as the LORD had said. Compare Exodus 4:21, Jehovah, because in connection with His word.
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.