Chāndogya Upaniṣad 7.5.2

7.5.2

tāni ha vā etāni cittaikāyanāni cittātmāni citte pratiṣṭhitāni \
tasmād yady api bahuvid acitto bhavati nāyam astīty evainam āhuḥ \
yad ayaṃ veda yad vā ayaṃ (S ) nettham acittaḥ syād iti \
atha yady alpavic cittavān bhavati tasmā evota śuśrūṣante \
cittaṃ hy evaiṣām ekāyanam \
cittam ātmā \
cittaṃ pratiṣṭhā \
cittam upāssveti \\705.2\

2. Verily, these things have Thought as their union-point, have Thought as their soul, are established on Thought. Therefore, even if one who knows much is without Thought, people say of him: “He is not anybody, whatever he knows! Verily, if he did know, he would not be so without Thought!” On the other hand, if one who knows little possesses Thought, people are desirous of listening to him. Truly, indeed, Thought is the union-point, Thought is the soul (_ātman_), Thought is the support of these things. Reverence Thought.