Katha-1-1-21-devairatrāpi

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

Death in order to test whether he was or was not absolutely fit to acquire the knowledge of the âtman 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 âtman is subtle. Therefore, Oh, Nachikêtas, ask for another boon, whose fruit is certain; press me not as a creditor presses a debtor. Give up this boon for me.

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