Katha-1-1-9-tisro rātrīryadavātsīrgṛhe

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

Thus addressed, Death having approached Nachikêtas worshipfully, said ‘as you have been living in my house fasting for three nights, a Brâhman guest worthy of reverence, therefore be my prostration to you. Oh Brâhman, therefore be good unto me and let me be freed from the sin of your having lived here fasting; although all good may befall me by your mere grace, still in order that I may propitiate you better, ask of me any three objects you wish for, one for every night you fasted.

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