Chandogya Upanishad 6.12

nyagrodhaphalam ata āhareti idaṃ bhagava iti bhinddhīti bhinnaṃ bhagava iti kim atra paśyasīti aṇvya ivemā dhānā bhagava iti āsām aṅgaikāṃ bhinddhīti bhinnā bhagava iti kim atra paśyasīti na kiṃcana bhagava iti

'Fetch me from thence a fruit of the Nyagrodha tree.' 'Here is one, Sir.' 'Break it.' 'It is broken, Sir.' 'What do you see there?' 'These seeds, almost infinitesimal.' 'Break one of them.' 'It is broken, Sir.' 'What do you see there?' 'Not anything, Sir.'

taṃ hovāca yaṃ vai somyaitam aṇimānaṃ na nibhālayasa etasya vai somyaiṣo 'ṇimna evaṃ mahānyagrodhas tiṣṭhati śraddhatsva somyeti

The father said: 'My son, that subtile essence which you do not perceive there, of that very essence this great Nyagrodha tree exists.

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

'Believe it, my son. That which is the subtile essence, in it all that exists has its self. It is the True. It is the Self, and thou, O Svetaketu, art it.' 'Please, Sir, inform me still more,' said the son. 'Be it so, my child,' the father replied.