Historical Rules · 23 April 2010
Rule 702.34d
Official source text from the 23 April 2010 Rules release
If you have priority, you may turn a face-down permanent you control face up. This is a special action; it doesn't use the stack (see rule 114). To do this, show all players what the permanent's morph cost would be if it were face up, pay that cost, then turn the permanent face up. (If the permanent wouldn't have a morph cost if it were face up, it can't be turned face up this way.) The morph effect on it ends, and it regains its normal characteristics. Any abilities relating to the permanent entering the battlefield don't trigger when it's turned face up and don't have any effect, because the permanent has already entered the battlefield.