702.37c

To cast a card using its morph ability, turn it face down and announce that you’re using a morph ability. It becomes a 2/2 face-down creature card with no text, no name, no subtypes, and no mana cost. Any effects or prohibitions that would apply to casting a card with these characteristics (and not the face-up card’s characteristics) are applied to casting this card. These values are the copiable values of that object’s characteristics. (See rule 613, “Interaction of Continuous Effects,” and rule 707, “Copying Objects.”) Put it onto the stack (as a face-down spell with the same characteristics), and pay {3} rather than pay its mana cost. This follows the rules for paying alternative costs. You can use a morph ability to cast a card from any zone from which you could normally cast it. When the spell resolves, it enters the battlefield with the same characteristics the spell had. The morph effect applies to the face-down object wherever it is, and it ends when the permanent is turned face up.