Source

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

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.

Rules

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

• 119.7. If an effect says that a player can’t gain life, that player can’t make an exchange such that the player’s life total would become higher; in that case, the exchange won’t happen. Similarly, if an effect redistributes life totals, a player can’t receive a new life total such that the player’s life total would become higher. In addition, a cost that involves having that player gain life can’t be paid, and a replacement effect that would replace a life gain event affecting that player won’t do anything.

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.