Rule 702.103a

Bestow represents a static ability that functions in any zone from which you could play the card it’s on. “Bestow [cost]” means “As you cast this spell, you may choose to cast it bestowed. If you do, you pay [cost] rather than its mana cost.” Casting a spell using its bestow ability follows the rules for paying alternative costs (see 601.2b and 601.2f–h).