Chandogya Upanishad 6.13.3

6.13.3

sa ya eṣo 'ṇimaitad ātmyam idaṃ sarvam \
tat satyam \
sa ātmā \
tat tvam asi śvetaketo iti \
bhūya eva mā bhagavān vijñāpayatv iti \
tathā somyeti hovāca \\613.3\

3. That which is the finest essence—this whole world has that as its soul. That is Reality. That is Ātman (Soul). That art thou, Śvetaketu.’ ‘Do you, Sir, cause me to understand even more.’ ‘So be it, my dear,’ said he.