Source

Source, in the context of Magic rules, can mean one of two things: source of an ability and source of damage.

Some effects from resolved spells and abilities prevent or replace damage from sources with certain properties, or characteristics, such as a creature or a source of a particular color.

Rules

From the Comprehensive Rules (June 7, 2024—Modern Horizons 3)

• 109.2c If a spell or ability uses a description of an object that includes the word “source,” it means a source matching that description—a source of an ability, of damage, or of mana—in any zone. See rules 113.7 and 609.7.

Source of an Ability

From the glossary of the Comprehensive Rules (June 7, 2024—Modern Horizons 3)

Source of an Ability

The object that generated that ability. See rule 113.7.

From the Comprehensive Rules (June 7, 2024—Modern Horizons 3)

• 113.7. The source of an ability is the object that generated it. The source of an activated ability on the stack is the object whose ability was activated. The source of a triggered ability (other than a delayed triggered ability) on the stack, or one that has triggered and is waiting to be put on the stack, is the object whose ability triggered. To determine the source of a delayed triggered ability, see rules 603.7d–f.
  • 113.7a Once activated or triggered, an ability exists on the stack independently of its source. Destruction or removal of the source after that time won’t affect the ability. Note that some abilities cause a source to do something (for example, “Prodigal Pyromancer deals 1 damage to any target”) rather than the ability doing anything directly. In these cases, any activated or triggered ability that references information about the source for use while announcing an activated ability or putting a triggered ability on the stack checks that information when the ability is put onto the stack. Otherwise, it will check that information when it resolves. In both instances, if the source is no longer in the zone it’s expected to be in at that time, its last known information is used. The source can still perform the action even though it no longer exists.

Source of Damage

From the glossary of the Comprehensive Rules (June 7, 2024—Modern Horizons 3)

Source of Damage

The object that dealt that damage. See rule 609.7.

From the Comprehensive Rules (June 7, 2024—Modern Horizons 3)

• 120.7. The source of damage is the object that dealt it. If an effect requires a player to choose a source of damage, they may choose a permanent; a spell on the stack (including a permanent spell); any object referred to by an object on the stack, by a prevention or replacement effect that’s waiting to apply, or by a delayed triggered ability that’s waiting to trigger (even if that object is no longer in the zone it used to be in); or a face-up object in the command zone. A source doesn’t need to be capable of dealing damage to be a legal choice. See rule 609.7, “Sources of Damage.”

From the Comprehensive Rules (June 7, 2024—Modern Horizons 3)

• 609.7. Some effects apply to damage from a source—for example, “The next time a red source of your choice would deal damage to you this turn, prevent that damage.”
  • 609.7a If an effect requires a player to choose a source of damage, they may choose a permanent; a spell on the stack (including a permanent spell); any object referred to by an object on the stack, by a replacement or prevention effect that’s waiting to apply, or by a delayed triggered ability that’s waiting to trigger (even if that object is no longer in the zone it used to be in); or a face-up object in the command zone. A source doesn’t need to be capable of dealing damage to be a legal choice. The source is chosen when the effect is created. If the player chooses a permanent, the effect will apply to the next damage dealt by that permanent, regardless of whether it’s combat damage or damage dealt as the result of a spell or ability. If the player chooses a permanent spell, the effect will apply to any damage dealt by that spell and any damage dealt by the permanent that spell becomes when it resolves.
  • 609.7b Some effects from resolved spells and abilities prevent or replace damage from sources with certain properties, such as a creature or a source of a particular color. When the source would deal damage, the “shield” rechecks the source’s properties. If the properties no longer match, the damage isn’t prevented or replaced. If for any reason the shield prevents no damage or replaces no damage, the shield isn’t used up.
  • 609.7c Some effects from static abilities prevent or replace damage from sources with certain properties. For these effects, the prevention or replacement applies to sources that are permanents with that property and to any sources that aren’t on the battlefield that have that property.