Historical Rules · 17 April 2020
Rule 605.3a
Official source text from the 17 April 2020 Rules release
A player may activate an activated mana ability whenever they have priority, whenever they are casting a spell or activating an ability that requires a mana payment, or whenever a rule or effect asks for a mana payment, even if it’s in the middle of casting or resolving a spell or activating or resolving an ability.