Historical Rules · 18 June 2010
Rule 605.4a
Official source text from the 18 June 2010 Rules release
A triggered mana ability doesn't go on the stack, so it can't be targeted, countered, or otherwise responded to. Rather, it resolves immediately after the mana ability that triggered it, without waiting for priority. Example: An enchantment reads, "Whenever a player taps a land for mana, that player adds one mana of that type to his or her mana pool." If a player taps lands for mana while casting a spell, the additional mana is added to the player's mana pool immediately and can be used to pay for the spell.