What can be said when applying ashes during Ash Wednesday Mass? For the first time in my life the minister in the town where I go to Mass while I'm at work said something besides "Remember your are dust and to dust you shall return" when administering ashes.  He said "Repent and believe in the Gospel" which is a good message, but makes me question 33 years of Ash Wednesdays, were we just being told 70's (because it conjures up Kansas' song "Dust in the Wind" in my mind at least) or what is the appropriate Ash Wednesday "thing to say"
The website liturgies.net appears to have a full copy of the Roman Missal, including the propers of the Mass for Ash Wednesday. This states:


  [The priest] sprinkles the ashes with holy water, without saying anything.
  
  Then the Priest places ashes on the head of all those present who come to him, and says to each one:
  
  Paenitemini, et credite Evangelion.
  Repent, and believe in the Gospel.
  
  Or:
  
  Memento, homo, quia pulvis es, et in pulverem reverteris.
  Remember that you are dust, and to dust you shall return.


I am attempting to find where in the Missal the statement originates.