Bag of Withholding

Whenever you tap a land for mana, put a storage
counter on that land. Whenever a land with a
storage counter is tapped for mana, that mana is
stored in Bag of Withholding instead. (Mana stored
in Bag of Withholding keep its color)

Pay 2 life: You gain X mana where X is the amount
of mana currently stored in Bag of Withholding.