Chandogya Upanishad 6.13

lavaṇam etad udake 'vadhāyātha mā prātar upasīdathā iti sa ha tathā cakāra taṃ hovāca yad doṣā lavaṇam udake 'vādhā aṅga tad āhareti taddhāvamṛśya na viveda

'Place this salt in water, and then wait on me in the morning.' The son did as he was commanded. The father said to him: 'Bring me the salt, which you placed in the water last night.' The son having looked for it, found it not, for, of course, it was melted.

yathā vilīnam eva aṅgāsyāntād ācāmeti katham iti lavaṇam iti madhyād ācāmeti katham iti lavaṇam iti antād ācāmeti katham iti lavaṇam iti abhiprāsyaitad atha mopasīdathā iti taddha tathā cakāra tacchaśvat saṃvartate taṃ hovācātra vāva kila tat somya na nibhālayase 'traiva kileti

The father said: 'Taste it from the surface of the water. How is it?' The son replied: 'It is salt.' 'Taste it from the middle. How is it?' The son replied: "It is salt.' 'Taste it from the bottom. How is it?' The son replied: 'It is salt.' The father said: 'Throw it away and then wait on me.' He did so; but salt exists for ever. Then the father said: 'Here also, in this body, forsooth, you do not perceive the True (Sat), my son; but there indeed it is.

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 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.