Historical Rules · 13 July 2018
Rule 701.33b
Official source text from the 13 July 2018 Rules release
Any time you have priority, you may turn a manifested permanent you control face up. This is a special action that doesn’t use the stack (see rule 115.2b). To do this, show all players that the card representing that permanent is a creature card and what that card’s mana cost is, pay that cost, then turn the permanent face up. The effect defining its characteristics while it was face down ends, and it regains its normal characteristics. (If the card representing that permanent isn’t a creature card or it doesn’t have a mana cost, it can’t be turned face up this way.)