Historical Rules · 17 April 2020
Rule 120.4a
Official source text from the 17 April 2020 Rules release
First, if an effect is causing damage to be dealt to a creature and that effect specifies that excess damage is dealt to another object or player instead, the damage event is modified to deal damage accordingly. Excess damage is damage greater than the creature’s toughness, taking into account damage marked on that creature and other damage being dealt to it at the same time as this damage. Any damage greater than 1 is excess damage if the source dealing that damage has deathtouch.