Chandogya Upanishad 6.11

asya somya mahato vṛkṣasya yo mūle 'bhyāhanyāj jīvan sraved yo madhye 'bhyāhanyāj jīvan sraved yo 'gre 'bhyāhanyāj jīvan sravet sa eṣa jīvenātmanānuprabhūtaḥ pepīyamāno modamānas tiṣṭhati

"If some one were to strike at the root of this large tree here, it would bleed, but live. If he were to strike at its stem, it would bleed, but live. If he were to strike at its top, it would bleed, but live. Pervaded by the living Self that tree stands firm, drinking in its nourishment and rejoicing;

asya yad ekāṃ śākhāṃ jīvo jahāty atha sā śuṣyati dvitīyāṃ jahāty atha sā śuṣyati tṛtīyāṃ jahāty atha sā śuṣyati sarvaṃ jahāti sarvaḥ śuṣyati

'But if the life (the living Self) leaves one of its branches, that branch withers; if it leaves at second, that branch withers; if it leaves a third, that branch withers. If it leaves the whole tree, the whole tree withers. In exactly the same manner, my son, know this.' Thus he spoke:

evam eva khalu somya viddhīti ha uvāca jīvāpetaṃ vāva kiledaṃ mriyate na jīvo mriyata iti 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

'This (body) indeed withers and dies when the living Self has left it; the living Self dies not. That which is that subtile essence, in it all that exists has its self. It is the True. It is the Self, and thou, Svetaketu, art it.' 'Please, Sir, inform me still more,' said the son. 'Be it so, my child,' the father replied.