Katha-1-1-10-śāntasaṃkalpaḥ

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

But Nachikêtas replies ‘if willing to grant boons, that my father be freed from anxiety, i.e., about me as to what his son would be doing after reaching Death, be calm in mind and not wrath against me; and again my father remember and believe me as the very son sent by him to you and sent home back by you and welcome me recognizing, Oh Death—is the first of the three boons I ask, the end of which is to gladden my father.’

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