Kaṭha Upaniṣad 51

51

devair apy atraitasmin vastuni vicikitsitaṃ saṃśayitaṃ purā pūrvam / na hi suvijñeyaṃ suṣṭhu jñeyam asakṛt śrutam api prākṛtair janair yato 'ṇuḥ sūkṣma eṣa ātmākhyo dharmaḥ / ato 'nyam asan digdhaphalaṃ varaṃ naciketo vṛṇīṣva, mā moparotsīr uparodhaṃ mā kārṣīr adharmaṇam ivottamarṇaḥ / atisṛja vimuñcainaṃ varaṃ mā māṃ prati ///1,1.21 //

Shankara’s Commentary:
Com.—Death in order to test whether he was or was not absolutely fit to acquire the knowledge of the atman leading to emancipation, said this: ‘Even by the gods in older times, doubt was entertained on this point. It is not easy to be known, though heard explained, by ordinary men. Because this subject of the atman is subtle. Therefore, Oh, Nachiketas, ask for another boon, whose fruit is certain; press me not as a creditor presses a debtor. Give up this boon for me.