The Gateless Gate, by Ekai, called Mu-mon, tr. Nyogen Senzaki and Paul Reps [1934], at sacred-texts.com
A monk asked Joshu why Bodhidharma came to China.
Joshu said: "An oak tree in the garden."
Mumon's comment: If one sees Joshu's answer clearly, there is no Shakyamuni Buddha before him and no future Buddha after him.