Bourn, Dreamer's Apprentice

{2}{u}{g}: Bourn, Dreamer's Apprentice has flying and hexproof until end of turn.
Whenever Bourn deals combat damage to a player, you may pay {2}{u}. If you do, turn the top card of your library face up. You may cast that card this turn without paying its mana cost if it's a nonland card. If not, exile it.