The Jataka, Vol. II, tr. by W.H.D. Rouse, , at sacred-texts.com
 "Tell me one word," etc.--This story the Master told in Jetavana, about a certain landowner.
We are told that there was a landowner who lived at Sāvatthi. One day, his son sitting on his hip asked him what is called the "Door 1" question. He replied, "That question requires a Buddha; nobody else can answer it." So he took his son to Jetavana, and saluted the Master. "Sir," said he, "as my son sat on my hip, he asked me the question called the 'Door.' I didn't know the answer, so here I am to ask you to give it." Said the Master, "This is not the first time, layman, that the lad has been a seeker after the way to accomplish his ends, and asked wise men this question; he did so before, and wise men in olden days gave him the answer; but by reason of the dimness caused by rebirth, he has forgotten it." And at his request the Master told a tale of the olden time.
Once upon a time, when Brahmadatta was reigning in Benares, the Bodhisatta came into this world as a rich merchant's son. He grew up, and when in course of time the father died, he took his father's place as a merchant.
And his son, a young boy, sitting on his hip, asked him a question, "Father," said he, "tell me a thing in one word which embraces a wide range of meaning;" and he repeated the first stanza:--
His father replied with the second:--
 Thus did the Bodhisatta answer his son's question. The son used the way which his father pointed out to accomplish his purposes, and by and bye he passed away to fare according to his deserts.
When this discourse was ended, the Master declared the Truths and identified the Birth:--at the conclusion of the Truths father and son reached the Fruit of the First Path:--"This man was then the son, and I was the merchant of Benares myself."
163:1 This question referred to the means of entering on the Paths.