Historical Rules · 18 June 2010
Rule 115.1d
Official source text from the 18 June 2010 Rules release
A player may activate a 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 in the middle of casting or resolving a spell or activating or resolving an ability).