Zigong asked, Is there a single word that can guide a person’s conduct throughout life?
The Master said, That would be reciprocity, wouldn’t it? What you do not want others to do to you, do not do to others.

[子貢問曰、有一言、而可以終身行之者乎。
子曰、其恕乎、己所不欲、勿施於人。]

Confucius (c. 551- c. 479 BC) Chinese philosopher, sage, politician [孔夫子 (Kǒng Fūzǐ, K'ung Fu-tzu, K'ung Fu Tse), 孔子 (Kǒngzǐ, Chungni), 孔丘 (Kǒng Qiū, K'ung Ch'iu)]

The Analects [論語, 论语, Lúnyǔ], Book 15, verse 24 (15.24) (6th C. BC – AD 3rd C.) [tr. Watson (2007)]
    (Source)

See also 5.12. Compare to the Bible, Matthew 7:12.

Legge and other earlier translators, as noted below, identified this as 15.23.

(Source (Chinese)). Alternate translations:

Tsze-kung asked, saying, "Is there one word which may serve as a rule of practice for all one's life?"
The Master said, "Is not RECIPROCITY such a word? What you do not want done to yourself, do not do to others."
[tr. Legge (1861), 15.23]

Tsz-kung put to him the question, "Is there one word upon which the whole life may proceed?" The Master replied, "Is not RECIPROCITY such a word? -- what you do not yourself desire, do not put before others."
[tr. Jennings (1895), 15.23. Jennings prefers translating shu as "like-heartedness" or "like-mindedness," but follows Legge.]

A disciple of Confucius enquired: "Is there one word which may guide one in practice throughout the whole life?"
Confucius answered, "The word 'charity' is perhaps the word. What you do not wish others to do unto you, do not do unto them."
[tr. Ku Hung-Ming (1898), 15.23]

"Is there any one word," asked Tzŭ Kung, "which could be adopted as a lifelong rule of conduct?"
The Master replied, "Is not Sympathy the word? Do not do to others what you would not like yourself."
[tr. Soothill (1910), 15.23; he translates shu in the notes as "the following of your good heart's prompting."]

Tze-kung asked if there were a single verb that you could practice through life up to the end.
He said: Sympathy, what you don't want (done to) yourself, don't inflict on another.
[tr. Pound (1933), 15.23]

Tzu-kung asked saying, Is there any single saying that one can act upon all day and every day?
The Master said, Perhaps the saying about consideration: "Never do to others what you would not like them to do to you."
[tr. Waley (1938), 15.23]

Tsekung asked, :Is there one single word that can serve as a principle or conduct for life?"
Confucius replied, "Perhaps the word "reciprocity" (shu) will do. Do not do unto others what you do not want others to do unto you."
[tr. Lin Yutang (1938); see also here and here.]

Tuan-mu Tz’u inquired, “Is there one word that will keep us on the path to the end of our days?”
“Yes. Reciprocity! What you do not wish yourself, do not unto others.”
[tr. Ware (1950)]

Tzu-kung asked, "Is there a single word which can be a guide to conduct throughout one's life?"
The Master said, "It is perhaps the word "shu." Do not impose on others what you yourself do not desire.
[tr. Lau (1979)]

Zigong asked: "Is there a single word such that one could practise it throughout one's life?"
The Master said: "Reciprocity perhaps? Do not inflict on others what you yourself would not wish done to you."
[tr. Dawson (1993)]

Zigong asked: "Is there any single word that could guide one's entire life?"
The Master said: "Should it not be reciprocity? What you do not wish for yourself, do not do to others."
[tr. Leys (1997)]

Zi-gong asked: “Is there one single word that one can practice throughout one’s life?”
The Master said: “It is perhaps ‘like-hearted considerateness.’ 'What you do not wish for yourself, do not impose on others.'"
[tr. Huang (1997)]

Zi-gong asked: "Is there one single word that one can practice throughout one's life?"
The Master said: "It is perhaps 'like-hearted considerateness.' 'What you do not wish for yourself, do not impose on others.'"
[tr. Huang (1997)]

Zigong asked, "Is there a word that can be practiced in all life?"
Confucius said: "It is the forgiveness. What is not wanted by oneself, should not be forced to others."
[tr. Cai/Yu (1998)]

Zigong asked, "Is there one expression that can be acted upon until the end of one's days?"
The Master replied "There is shu: do not impose on others what you yourself do not want."
[tr. Ames/Rosemont (1998)]

Dž-gùng asked, is there one saying that one can put in practice in all circumstances?
The Master said, That would be empathy, would it not? What he himself does not want, let him not do it to others.
[tr. Brooks/Brooks (1998)]

Adept Kung asked: "Is there any one word that could guide a person throughout life?"
The Master replied, "How about "shu": never impose on others what you would not choose for yourself.
[tr. Hinton (1998)]

Zigong asked, “Is there one word that can serve as a guide for one’s entire life?”
The Master answered, “Is it not ‘understanding’ ? Do not impose upon others what you yourself do not desire.”
[tr. Slingerland (2003)]

Zigong asked: "Is there a single word that can serve as the guide to conduct throughout one's life?"
The Master said, "It is perhaps the word shu. Do not impose on others what you yourself do not want."
[tr. Chin (2014); Chin translates shu as "treating others with an awareness that they, too, are alive with humanity"]