KAṬHA UPANISHAD · 2.2.6

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

hanta ta idaṃ pravakṣyāmi guhyaṃ brahma sanātanam ।
yathā ca maraṇaṃ prāpya ātmā bhavati gautama ॥ 6॥

Surely, O Gautama, I will tell thee of this secret and eternal Brahman and likewise what becometh of the soul when one dieth.