Historical Rules · 1 May 2014
Rule 112.3b
Official source text from the 1 May 2014 Rules release
Activated abilities have a cost and an effect. They are written as “[Cost]: [Effect.] [Activation instructions (if any).]” A player may activate such an ability whenever he or she has priority. Doing so puts it on the stack, where it remains until it’s countered, it resolves, or it otherwise leaves the stack. See rule 602, “Activating Activated Abilities.”