Katha-1-2-13-etacchrutvā

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

Again having heard this, the true âtman which I shall explain to you—from the presence of the preceptor and well-grasped it as his own Self, having abstracted the virtuous âtman from the body, etc., and having realized this subtle âtman, the learned mortal rejoices having obtained what gives him joy, i.e., the âtman. The door of such abode of Brahman is, I think, wide open for you, Nachikêtas. The drift is ‘I think you worthy of emancipation.’

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