Who told Isaiah to walk around naked and barefoot for three years?

God told him to

(for a sign and a wonder)

At the same time spake the LORD by Isaiah the son of Amoz, saying, Go and loose the sackcloth from off thy loins, and put off thy shoe from thy foot. And he did so, walking naked and barefoot.
And the LORD said, Like as my servant Isaiah hath walked naked and barefoot three years for a sign and wonder. Isaiah 20:2-3