Historical Rules · 1 July 2012
Rule 106.4a
Official source text from the 1 July 2012 Rules release
If a player passes priority (see rule 116) 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.