Katha Upanishad Verse 2.2.6

Yama addresses Nachiketas (as Gautama) with the promise to reveal the ultimate mystery: the “secret ancient Brahman.” He intends to explain not just the nature of the absolute reality, but also the specific condition and state of the Atman after it has undergone the experience of physical death.

Sanskrit Verse

हन्त त इदं प्रवक्श्यामि गुह्यं ब्रह्म सनातनम् । यथा च मरणं प्राप्य आत्मा भवति गौतम ॥ ६ ॥

Transliteration

hanta ta idaṃ pravakśyāmi guhyaṃ brahma sanātanam | yathā ca maraṇaṃ prāpya ātmā bhavati gautama || 6 ||

Translation

To thee, Oh Gautama, I will explain the secret ancient Brahman and also how after death, the atman becomes.