Historical Rules · 1 February 2010
Rule 112.7
Official source text from the 1 February 2010 Rules release
The source of an ability on the stack is the object that generated that ability. For an activated ability, it's the object whose ability was activated. For a triggered ability (other than a delayed triggered ability), it's the object whose ability triggered. For a delayed triggered ability created by a spell, it's that spell. For a delayed triggered ability created by another ability, it's the source of that other ability.