Isaiah 6:8 (KJV)

Then flew one of the seraphims unto me, having a live coal in his hand, which he had taken with the tongs from off the altar:

And he laid it upon my mouth, and said, Lo, this hath touched thy lips; and thine iniquity is taken away, and thy sin purged.

Also I heard the voice of the Lord, saying, Whom shall I send, and who will go for us? Then said I, Here am I; send me.

And he said, Go, and tell this people, Hear ye indeed, but understand not; and see ye indeed, but perceive not.

Make the heart of this people fat, and make their ears heavy, and shut their eyes; lest they see with their eyes, and hear with their ears, and understand with their heart, and convert, and be healed.