Chandogya Upanishad 6.7

ṣoḍaśakalaḥ somya puruṣaḥ pañcadaśāhāni māśīḥ kāmam apaḥ piba āpomayaḥ prāṇo na pibato vicchetsyata iti

'Man (purusha), my son, consists of sixteen parts. Abstain from food for fifteen days, but drink as much water as you like, for breath comes from water, and will not be cut off, if you drink water.'

sa ha pañcadaśāhāni nāśa atha hainam upasasāda kiṃ bravīmi bho iti ṛcaḥ somya yajūṃṣi sāmānīti sa hovāca na vai mā pratibhānti bho iti

Svetaketu abstained from food for fifteen days. Then he came to his father and said: 'What shall I say?' The father said: 'Repeat the Rik, Yagus, and Sâman verses.' He replied: 'They do not occur to me, Sir.'

taṃ hovāca yathā somya mahato 'bhyāhitasyaiko 'ṅgāraḥ khadyotamātraḥ pariśiṣṭaḥ syāt tena tato 'pi na bahu dahet evaṃ somya te ṣoḍaśānāṃ kalānām ekā kalā atiśiṣṭā syāt tayaitarhi vedān nānubhavasi aśāna atha me vijñāsyasīti

The father said to him: 'As of a great lighted fire one coal only of the size of a firefly may be left, which would not burn much more than this (i.e. very little), thus, my dear son, one part only of the sixteen parts (of you) is left, and therefore with that one part you do not remember the Vedas. Go and eat!

sa hāśa atha hainam upasasāda taṃ ha yat kiṃca papraccha sarvaṃ ha pratipede

'Then wilt thou understand me.' Then Svetaketu ate, and afterwards approached his father. And whatever his father asked him, he knew it all by heart. Then his father said to him:

taṃ hovāca yathā somya mahato 'bhyāhitasyaikam aṅgāraṃ khadyotamātraṃ pariśiṣṭaṃ taṃ tṛṇair upasamādhāya prajvālayet tena tato 'pi bahu dahet

'As of a great lighted fire one coal of the size of a firefly, if left, may be made to blaze up again by putting grass upon it, and will thus burn more than this,

evaṃ somya te ṣoḍaśānāṃ kalānām ekā kalātiśiṣṭābhūt sānnenopasamāhitā prājvālī tayaitarhi vedān anubhavasi annamayaṃ hi somya manaḥ āpomayaḥ prāṇaḥ tejomayī vāg iti taddhāsya vijajñāv iti vijajñāv iti

'Thus, my dear son, there was one part of the sixteen parts left to you, and that, lighted up with food, burnt up, and by it you remember now the Vedas.' After that, he understood what his father meant when he said: 'Mind, my son, comes from food, breath from water, speech from fire.' He understood what he said, yea, he understood it.