Historical Rules · 25 August 2017
Rule 118.8
Official source text from the 25 August 2017 Rules release
If an effect says that a player can’t lose life, that player can’t make an exchange such that the player’s life total would become lower; in that case, the exchange won’t happen. Similarly, if an effect redistributes life totals, a player can’t receive a new life total such that the player’s life total would become lower. In addition, a cost that involves having that player pay life can’t be paid.