Historical Rules · 18 June 2010
Rule 106.4a
Official source text from the 18 June 2010 Rules release
If a player passes priority (see rule 115) while there is mana in his or her mana pool, that player announces what mana is there. If any mana remains in a player's mana pool after he or she spends mana to pay a cost, that player announces what mana is still there.