Kaṭha Upaniṣad 2.8

2.8

na nareṇāvareṇa prokta eṣa suvijñeyo bahudhā cintyamānaḥ /
ananyaprokte gatir atra nāsty aṇīyān hy atarkyam aṇupramāṇāt //

8. Not, when proclaimed by an inferior man, is He
To be well understood, [though] being manifoldly considered.
Unless declared by another, there is no going thither;
For He is inconceivably more subtile than what is of subtile
measure.