Where does it say 'You Are Already Enlightened'?
It's a basic Mahayana doctrine, important in other schools as well as Zen. 


  Hongaku is an East Asian Buddhist doctrine often translated as
  "inherent", "innate", "intrinsic" or "original" enlightenment... It is
  first mentioned in the Awakening of Faith in the Mahayana scripture.


https://en.wikipedia.org/wiki/Hongaku

Wikipedia also says it can be traced back to sayings of the Buddha in the Anguttara Nikaya: https://en.wikipedia.org/wiki/Buddha-nature#Earliest_sources