The Gateless Gate, by Ekai, called Mu-mon, tr. Nyogen Senzaki and Paul Reps [1934], at sacred-texts.com
Two monks were arguing about a flag. One said: "The flag is moving."
The other said: "The wind is moving."
The sixth patriarch happened to be passing by. He told them: "Not the wind, not the flag; mind is moving."
Mumon's comment: The sixth patriarch said: "The wind is not moving, the flag is not moving. Mind is moving." What did he mean? If you understand this intimately, you will see the two monks there trying to buy iron and gaining gold. The sixth patriarch could not bear to see those two dull heads, so he made such a bargain.