Rule 702.75a

Prowl is a static ability that functions on the stack. "Prowl [cost]" means "You may pay [cost] rather than pay this spell's mana cost if a player was dealt combat damage this turn by a source that, at the time it dealt that damage, was under your control and had any of this spell's creature types." Paying a spell's prowl cost follows the rules for paying alternative costs in rules 601.2b and 601.2f-h.

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