Brihadaranyaka Upanishad 3.4.1

3.4.1

atha hainamuṣastaścākrāyaṇaḥ papraccha yājñavalkyeti hovāca yatsākṣādaparokṣādbrahma ya ātmā sarvāntarastaṃ me vyācakṣva ityeṣa ta ātmā sarvāntaraḥkatamo yājñavalkya sarvāntaro yaḥ prāṇena prāṇiti sa ta ātmā sarvāntaroyo'pānenāpānīti sa ta ātmā sarvāntaro yo vyānena vyānīti sa ta ātmā sarvāntaro ya udānenodāniti sa ta ātmā sarvāntara eṣa ta ātmāsarvāntaraḥ

Then Ushasta Kâkrâyana asked. 'Yâgñavalkya,' he said, 'tell me the Brahman which is visible, not invisible, the Self (âtman), who is within all.'
Yâgñavalkya replied: 'This, thy Self, who is within all.'
'Which Self, O Yâgñavalkya, is within all?'
Yâgñavalkya replied: 'He who breathes in the up-breathing, he is thy Self, and within all. He who breathes in the down-breathing, he is thy Self, and within all. He who breathes in the on-breathing, he is thy Self, and within all. He who breathes in the out-breathing, he is thy Self, and within all. This is thy Self, who is within all.'