The Gateless Gate, by Ekai, called Mu-mon, tr. Nyogen Senzaki and Paul Reps [1934]
A Zen student told Ummon: "Brilliancy of Buddha illuminates the whole universe."
Before he finished the phrase Ummon asked: "You are reciting another's poem, are you not?"
"Yes," answered the student.
"You are sidetracked," said Ummon.
Afterwards another teacher, Shishin, asked his pupils: "At what point did that student go off the track?"
Mumon's comment: If anyone perceives Ummon's particular skillfulness, he will know at what point the student was off the track, and he will be a teacher of man and Devas. If not, he cannot even perceive himself.