Historical Rules · 29 September 2017
Rule 605.3a
Official source text from the 29 September 2017 Rules release
A player may activate an activated mana ability whenever he or she has priority, whenever he or she is 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.