Historical Rules · 13 July 2013
Rule 112.5
Official source text from the 13 July 2013 Rules release
Some activated abilities are loyalty abilities. Loyalty abilities follow special rules: A player may activate a loyalty ability of a permanent he or she controls any time he or she has priority and the stack is empty during a main phase of his or her turn, but only if no player has previously activated a loyalty ability of that permanent that turn. See rule 606, "Loyalty Abilities."