Who told Perseus to decapitate Medusa?

There are several versions of the myth.

In one, Polydectes, the ruler of the island that Perseus and his mother, Danaë lived one, desperately wanted to marry Danaë, but didn't want her son around. Hence, he hatched a plot in which he would invite a bunch of people over for a wedding celebration, including Perseus. As dictated by custom, everyone brought a gift for the bride-to-be, except for Perseus, since he had nothing to give. He was deeply embarrassed by this, and declared that he would give the king anything he wanted. Therefore, the king told Perseus that he wanted nothing more than the head of Medusa.

In another version (but at the same wedding celebration), Perseus declared that he himself would get the head of Medusa as a gift for Polydectes.

In both, Polydectes was hoping that Perseus would not make it back alive.