Aha, here it is.
502.22a Flashback is a static ability of some instant and sorcery cards that functions while the card is in a player’s graveyard. “Flashback [cost]” means “You may play this card from your graveyard by paying [cost] rather than paying its mana cost. If you do, remove this card from the game instead of putting it anywhere else any time it would leave the stack.” Playing a spell using its flashback ability follows the rules for paying alternative costs in rules 409.1b and 409.1f–h.