Historical Rules · 25 January 2019
Rule 119.6
Official source text from the 25 January 2019 Rules release
Damage marked on a creature remains until the cleanup step, even if that permanent stops being a creature. If the total damage marked on a creature is greater than or equal to its toughness, that creature has been dealt lethal damage and is destroyed as a state-based action (see rule 704). All damage marked on a permanent is removed when it regenerates (see rule 701.14, "Regenerate") and during the cleanup step (see rule 514.2).