Historical Rules · 23 April 2010
Rule 112.7
Official source text from the 23 April 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. To determine the source of a delayed triggered ability, see rules 603.7d-f.