Speaking of Yen Hui, the Master said: “How sad — to watch him forge ahead so resolutely, and never see how far he could go.”

[子謂顏淵曰、惜乎、吾見其進也、未見其止也。]

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 9, verse 21 (9.21) (6th C. BC – AD 3rd C.) [tr. Hinton (1998)]
    (Source)

Regarding his finest student, who died young. Probable origin of a phrase frequently attributed to Confucius: "It does not matter how slowly you go so long as you do not stop."

Earlier sources use Legge's numbering of 9.20, as noted. (Source (Chinese)). Alternate translations:

The Master said of Yen Yuan, "Alas! I saw his constant advance. I never saw him stop in his progress."
[tr. Legge (1861), 9.20]

"Alas for Hwúi! I saw him (ever) making progress. I never saw him stopping short."
[tr. Jennings (1895), 9.20]

Confucius remarked of the same disciple [Yen Hui]: "Alas! he is dead. I have observed his constant advance; I never saw him stop in his progress."
[tr. Ku Hung-Ming (1898), 9.20]

The Master, referring to Yen Yüan, said: "Alas! I ever saw him make progress, and never saw him stand still."
[tr. Soothill (1910), 9.20]

He described Yen Yuan: Alas, I see him advance, I never see him stop (take a position).
[tr. Pound (1933), 9.20]

The Master said of Yen Hui, Alas, I saw him go forward, but had no chance to see whither this progress would have led him in the end.
[tr. Waley (1938), 9.20]

What a pity that Yen Hui is gone! I saw him make progress, but I never found out what his limit was.
[tr. Ware (1950)]

Alas! I only saw him advance and never saw him stop.
[tr. Huang (1997)]

The Master said of Yen Yūan, "I watched him making progress, but I did not see him realize his capacity to the full. What a pity!"
[tr. Lau (1979)]

The master said of Yan Hui: "Alas! I saw that he was making progress, but I never saw that he was stopping."
[tr. Dawson (1993)]

The Master said of Yan Hui: "Alas, I watched his progress, but did not see him reach the goal."
[tr. Leys (1997)]

The master, speaking of Yan Yuan, said: "Alas! I only saw him advance and never saw him stop."
[tr. Huang (1997)]

Confucius talked about Yuan Yan, said: "What regrettable it is! I just saw that he kept going ahead, and never saw that he got any stopping."
[tr. Cai/Yu (1998), #230]

The Master said about Yan Hui, "Such a pity! I only saw his progress; I never saw where he got to."
[tr. Ames/Rosemont (1998)]

The Master said of Yén Hwéi, Alas! I saw him start, but I did not see him finish.
[tr. Brooks/Brooks (1998)]

The Master said of Yan Hui, “Alas! I watched his advance, and never once saw him stop.”
[tr. Slingerland (2003)]

Speaking of Yan Yuan, the Master said, What a pity! I saw him move forward. I never saw him come to a stop.
[tr. Watson (2007)]

The Master, referring to Yan Hui, said, "It is a pity! I saw him moving forward but did not see him complete his journey."
[tr. Chin (2014)]

Confucius said of Yan Yuan, "Impressive indeed! I always see his progress and have never noticed his pause."
[tr. Li (2020)]