Historical Rules · 1 February 2010
Rule 117.4
Official source text from the 1 February 2010 Rules release
If a cost or effect allows a player to pay an amount of life greater than 0, the player may do so only if his or her life total is greater than or equal to the amount of the payment. If a player pays life, the payment is subtracted from his or her life total; in other words, the player loses that much life. (Players can always pay 0 life.)