Chandogya Upanishad 6.10

imāḥ somya nadyaḥ purastāt prācyaḥ syandante paścāt pratīcyaḥ tāḥ samudrāt samudram evāpiyanti sa samudra eva bhavati tā yathā tatra na vidur iyam aham asmīyam aham asmīti

'These rivers, my son, run, the eastern (like the Gangâ) toward the east, the western (like the Sindhu) toward the west. They go from sea to sea (i.e. the clouds lift up the water from the sea to the sky, and send it back as rain to the sea). They become indeed sea. And as those rivers, when they are in the sea, do not know, I am this or that river,

evam eva khalu somyemāḥ sarvāḥ prajāḥ sata āgamya na viduḥ sata āgacchāmaha iti ta iha vyāghro vā siṃho vā vṛko vā varāho vā kīṭo vā pataṅgo vā daṃśo vā maśako vā yad yad bhavanti tad ābhavanti

'In the same manner, my son, all these creatures, when they have come back from the True, know not that they have come back from the True. Whatever these creatures are here, whether a lion, or a wolf, or a boar, or a worm, or a midge, or a gnat, or a musquito, that they become again and again.

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

'That which is that 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.