DoesItResolve Unofficial Magic rules reference
In this rule

Historical Rules · 1 October 2010

Rule 603.3a

Official source text from the 1 October 2010 Rules release

PermalinkView evolution

A triggered ability is controlled by the player who controlled its source at the time it triggered, unless it's a delayed triggered ability. To determine the controller of a delayed triggered ability, see rules 603.7d­f.

Find this number in the current Rules