Kaṭha Upaniṣad 88

88

na nareṇāvareṇa prokta eṣa suvijñeyo bahudhā cintyamānaḥ /
ananyaprokte gatir atra nāsty aṇīyān hy atarkyam aṇupramāṇāt ///1,2.8 //

8. This atman now explained cannot easily be known, of taught by a person of inferior intellect being variously regarded. When it is taught by a preceptor, one with the Brahman, there is no further travel, his being subtler than the subtle and not arguable.