Confucius. Book 2, Chapter 9.
Book 2, Wei Chang
The Master said, "I have talked with Hui for a whole day, and he has not made any objection to anything I said; ‹ as if he were stupid. He has retired, and I have examined his conduct when away from me, and found him able to illustrate my teachings. Hui! ‹ He is not stupid."
Text source: Lun Yu, The Confucian Analects. Translated by James Legge.