Historical Rules · 1 May 2013
Rule 116.1d
Official source text from the 1 May 2013 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).