Rhystic Study

Oracle Text

Whenever an opponent casts a spell, you may draw a card unless that player pays 1.

Card Rulings

10/4/2004 The player gets the option to pay when this triggered ability resolves. This will be after the spell is announced, but before it resolves.
10/4/2004 You don't have to decide whether or not you are drawing until after the player decides whether or not to pay.