Kaṭha Upaniṣad 52

52

evam ukto naciketā āha --

Shankara’s Commentary:
Com.—Thus addressed, Nachiketas said, ‘I have heard from yourself that even the gods had doubt on this point and you say, Oh Death, that the true nature of the atman is not easy to know. As this cannot be known even by the learned, another, learned like you 2 to explain this, cannot be found, though sought.’ This boon also is a means to the attainment of emancipation and there is no other boon which can at all weigh with this. The meaning is that other boons bear only transitory fruits.