Rule 801.15b

If a spell or ability creates an effect that prevents damage that would be dealt by a source, it can affect only sources within the spell or ability's controller's range of influence. If a spell or ability creates an effect that prevents damage that would be dealt to a creature or player, it can affect only creatures and players within the spell or ability's controller's range of influence. If a spell or ability creates an effect that prevents damage, but neither the source nor the would-be recipient of the damage is specified, it prevents damage only if both the source and recipient of that damage are within the spell or ability's controller's range of influence. Example: Rob is within Alex's range of influence, but Carissa is not. Alex controls an enchantment that says, "Prevent all damage that would be dealt by creatures." Carissa attacks Rob with a creature. The creature deals combat damage to Rob. Example: Rob is within Alex's range of influence, but Carissa is not. Carissa casts Lightning Blast ("Lightning Blast deals 4 damage to target creature or player") targeting Rob. In response, Alex casts Mending Hands ("Prevent the next 4 damage that would be dealt to target creature or player this turn") targeting Rob. The damage to Rob is prevented. Example: Rob is within Alex's range of influence, but Carissa is not. Carissa attacks Rob with a creature, and Rob blocks with a creature. Alex casts Holy Day ("Prevent all combat damage that would be dealt this turn.") Carissa and Rob's creatures deal combat damage to each other.

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