Delayed Blast Fireball
Instant
Delayed Blast Fireball deals 2 damage to each opponent and each creature they control. If this spell was cast from exile, it deals 5 damage to each opponent and each creature they control instead.
Foretell 
(During your turn, you may pay 
and exile this card from your hand face down. Cast it on a later turn for its foretell cost.)
Andreas Zafiratos
Rulings
Because exiling a card with foretell from your hand is a special action, you can do so any time you have priority during your turn, including in response to spells and abilities. Once you announce you're taking the action, no other player can respond by trying to remove the card from your hand.
(wotc • 2022-06-10)
Delayed Blast Fireball deals 5 damage if it was cast from exile for any reason; it doesn't have to be from its foretell ability.
(wotc • 2022-06-10)
If you're casting a foretold card from exile for its foretell cost, you can't choose to cast it for any other alternative costs. You can, however, pay additional costs, such as kicker costs. If the card has any mandatory additional costs, those must be paid to cast the spell.
(wotc • 2022-06-10)
Casting a foretold card from exile follows the timing rules for that card. If you foretell an instant card, you can cast it as soon as the next player's turn. In most cases, if you foretell a card that isn't an instant (or doesn't have flash), you'll have to wait until your next turn to cast it.
(wotc • 2022-06-10)
Related Cards
Printings
Rulings
Because exiling a card with foretell from your hand is a special action, you can do so any time you have priority during your turn, including in response to spells and abilities. Once you announce you're taking the action, no other player can respond by trying to remove the card from your hand.
(wotc • 2022-06-10)
Delayed Blast Fireball deals 5 damage if it was cast from exile for any reason; it doesn't have to be from its foretell ability.
(wotc • 2022-06-10)
If you're casting a foretold card from exile for its foretell cost, you can't choose to cast it for any other alternative costs. You can, however, pay additional costs, such as kicker costs. If the card has any mandatory additional costs, those must be paid to cast the spell.
(wotc • 2022-06-10)
Casting a foretold card from exile follows the timing rules for that card. If you foretell an instant card, you can cast it as soon as the next player's turn. In most cases, if you foretell a card that isn't an instant (or doesn't have flash), you'll have to wait until your next turn to cast it.
(wotc • 2022-06-10)