Hesiod. Works and Days. (Lines 83-89).
But when he had finished the sheer, hopeless snare, the Father sent glorious Argus-Slayer, the swift messenger of the gods, to take it to Epimetheus as a gift. And Epimetheus did not think on what Prometheus had said to him, bidding him never take a gift of Olympian Zeus, but to send it back for fear it might prove to be something harmful to men. But he took the gift, and afterwards, when the evil thing was already his, he understood.