Shoyoroku - Case 46: Tokusan's "Study Accomplished"

Great Master Tokusan Emmyo instructed his assembly and said,
  "If you have exhausted to the end, you will realize right away that all
   buddhas in the three worlds have stuck their mouths to the wall [1].
   Yet there is still one person ­ he is giving a great laugh. If you can
   recognize that person, you have accomplished your study."

[1]: I.e., they are unable to open their mouths.