Historical Rules · 20 January 2017
Rule 107.5
Official source text from the 20 January 2017 Rules release
The tap symbol is {T}. The tap symbol in an activation cost means “Tap this permanent.” A permanent that’s already tapped can’t be tapped again to pay the cost. A creature’s activated ability with the tap symbol in its activation cost can’t be activated unless the creature has been under its controller’s control continuously since his or her most recent turn began. See rule 302.6.