Kaṭha Upaniṣad 86

86

śravaṇāyāpi bahubhir yo na labhyaḥ śṛṇvanto 'pi bahavo yaṃ na vidyuḥ /
āścaryo vaktā kuśalo 'sya labdhā āścaryo jñātā kuśalānuśiṣṭaḥ ///1,2.7 //

7. Who cannot be attained even for hearing by many; whom, many though hearing, do not know; the expounder of him is a wonder; and able, the attainer of him; a wonder, the knower of him instructed by the able.