Confucius. Book 2, Chapter 17.
Book 2, Wei Chang
The Master said, "Yu, shall I teach you what knowledge is? When you know a thing, to hold that you know it; and when you do not know a thing, to allow that you do not know it; ‹ this is knowledge."
Text source: Lun Yu, The Confucian Analects. Translated by James Legge.