Text:

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

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

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

Shankara’s Commentary:

I will now explain to thee again this secret ancient Brahman by whose knowledge cessation of all samsara results and not knowing which, how after death the atman travels in samsara. Listen, Oh Gautama.