Muṇḍaka Upaniṣad 3.2.3

3.2.3

nāyam ātmā pravacanena labhyo na medhayā na bahunā śrutena.
yam evaiṣa vṛṇute tena labhyas tasyaiṣa ātmā vivṛṇute tanūṃ svām.

3. This Soul (Ātman) is not to be obtained by instruction,
Nor by intellect, nor by much learning.
He is to be obtained only by the one whom He chooses;
To such a one that Soul (Ātman) reveals His own person
(*tanūm svām*).