Historical Rules · 12 July 2019
Rule 119.4
Official source text from the 12 July 2019 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 their life total is greater than or equal to the amount of the payment. If a player pays life, the payment is subtracted from their life total; in other words, the player loses that much life. (Players can always pay 0 life.)