Kaṭha Upaniṣad 50

50

devair atrāpi vicikitsitaṃ purā na hi sujñeyam aṇur eṣa dharmaḥ /
anyaṃ varaṃ naciketo vṛṇīṣva mā moparotsīr ati mā sṛjainam ///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.