Katha-1-1-15-lokādimagniṃ

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

This, the Sruti itself says. Death explained o Nachikêtas, the fire, which was the source of the worlds, being the first embodied existence, the same that was asked for by Nachikêtas; again he told him of what form the sacrificial bricks were to be, how many in number and in what manner the sacrificial fire was to be lit and all this; Nachikêtas also faithfully repeated what was told him by Death. Then delighted by his repetition, Death said to him again inclined to grant him a boon other than the three promised.

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