Katha-1-1-22-devairatrāpi

Sri Shankara’s Commentary (Bhashya) translated by S. Sitarama Sastri

Thus addressed, Nachikêtas 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 âtman 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.

Links
Various resources on the Kathopanishad
Upanishad Series – Katopanishad Commentary by Swami Chinmayananda