What rule did Dustin Johnson violate?

HAVEN, Wis. -- *Dustin Johnson's two-stroke penalty in the final round of the PGA Championship is for violating Rule 13-4: Ball in Hazard; Prohibited Action. *

Except as provided in the Rules, before making a stroke at a ball that is in a hazard (whether a bunker or a water hazard) or that, having been lifted from a hazard, may be dropped or placed in the hazard, the player must not:

The Supplementary Rules of Play distributed to players this week also included this advistory:

-- Sam Weinman