Rule 702.102c

The check for whether a spell can legally be cast happens after its controller has chosen whether to pay its bestow cost; see rule 601.2e. Example: Aether Storm is an enchantment with the ability “Creature spells can’t be cast.” This effect doesn’t stop a creature card with bestow from being cast for its bestow cost because the spell is an Aura enchantment spell, not an enchantment creature spell, when the game checks whether the spell is illegal.

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