Rule 117.12

Some spells, activated abilities, and triggered abilities read, "[Do something]. If [a player] [does or doesn't], [effect]." or "[A player] may [do something]. If [that player] [does or doesn't], [effect]." The action [do something] is a cost, paid when the spell or ability resolves. The "If [a player] [does or doesn't]" clause checks whether the player chose to pay an optional cost or started to pay a mandatory cost, regardless of what events actually occurred. Example: You control Hesitation, an enchantment that says "When a player casts a spell, sacrifice Hesitation. If you do, counter that spell." A spell is cast, causing Hesitation's ability to trigger. Then an ability is activated that exiles Hesitation. When Hesitation's ability resolves, you're unable to pay the "sacrifice Hesitation" cost. The spell is not countered. Example: Your opponent has cast Gather Specimens, a spell that says "If a creature would enter the battlefield under an opponent's control this turn, it enters the battlefield under your control instead." You control a face-down Dermoplasm, a creature with morph that says "When Dermoplasm is turned face up, you may put a creature card with morph from your hand onto the battlefield face up. If you do, return Dermoplasm to its owner's hand." You turn Dermoplasm face up, and you choose to put a creature card with morph from your hand onto the battlefield. Due to Gather Specimens, it enters the battlefield under your opponent's control instead of yours. However, since you chose to pay the cost, Dermoplasm is still returned to its owner's hand.

Find this number in the current Rules