The Gateless Gate, by Ekai, called Mu-mon, tr. Nyogen Senzaki and Paul Reps , at sacred-texts.com
Gutei raised his finger whenever he was asked a question about Zen. A boy attendant began to imitate him in this way. When anyone asked the boy what his master had preached about, the boy would raise his finger.
Gutei heard about the boy's mischief. He seized him and cut off his finger. The boy cried and ran away. Gutei called and stopped him. When the boy turned his head to Gutei, Gutei raised up his own finger. In that instant the boy was enlightened.
When Gutei was about to pass from this world he gathered his monks around him. "I attained my finger-Zen," he said, "from my teacher Tenryu, and in my whole life I could not exhaust it." Then he passed away.
Mumon's comment: Enlightenment, which Gutei and the boy attained, has nothing to do with a finger. If anyone clings to a finger, Tenryu will be so disappointed that he will annihilate Gutei, the boy, and the clinger all together.