Bṛhadāraṇyaka Upaniṣad ("Great Forest Teaching") 3.7.1

3.7.1

atha hainamuddālaka āruṇiḥ papraccha yājñavalkyeti hovāca madreṣvavasāma patañcalasya kāpyasya gṛheṣu yajñamadhīyānāstasyāsīdbhāryā gandharvagṛhītā tamapṛcchāma ko'sīti so'bravīt kabandha ātharvaṇa iti so'bravītpatañcalaṃ kāpyaṃ yājñikām̐śca vettha nu tvaṃ kāpya tatsūtraṃ yenāyaṃ ca lokaḥ paraśca lokaḥsarvāṇi ca bhūtāni saṃdṛbdhāni bhavantīti so'bravītpatañcalaḥ kāpyo nāhaṃtadbhagavanvedeti so'bravītpatañcalaṃ kāpyaṃ yājñikām̐śca vettha nu tvaṃ kāpya tamantaryāmiṇaṃ ya imaṃ ca lokaṃ paraṃ ca lokam̐ sarvāṇi ca bhūtāni yo'ntaro yamayatīti so'bravītpatañcalaḥ kāpyo nāhaṃ taṃ bhagavanvedeti so'bravītpatañcalaṃ kāpyaṃ yājñikām̐śca yo vaitatkāpya sūtraṃ vidyāttaṃ cāntaryāmiṇamiti sa brahmavitsalokavitsa devavitsa vedavitsa bhūtavitsa ātmavitsa sarvaviditi tebhyo'bravittadahaṃ veda taccettvaṃ yājñavalkya sūtramavidvām̐staṃ cāntaryāmiṇaṃ brahmagavīrudajase mūrdhā te vipatiṣyatīti veda vā ahaṃ gautama tatsūtraṃ taṃ cāntaryāmiṇamiti yo vā idaṃ kaścidbabrūyādveda vedeti yathā vettha tathā brūhīti

Then Uddâlaka Âruni asked. 'Yâgñavalkya,' he said, 'we dwelt among the Madras in the houses of Patañkala Kâpya, studying the sacrifice. His wife was possessed of a Gandharva, and we asked him: "Who art thou?" He answered: "I am Kabandha Âtharvana." And he said to Patañkala Kâpya and to (us) students: "Dost thou know, Kâpya, that thread by which this world and the other world, and all beings are strung together?" And Patañkala Kâpya replied: "I do not know it, Sir." He said again to Patañkala Kâpya and to (us) students: "Dost thou know, Kâpya, that puller (ruler) within (antaryâmin), who within pulls (rules) this world and the other world and all beings?" And Patañkala Kâpya replied: "I do not know it, Sir." He said again to Patañkala Kâpya and to (us) students: "He, O Kâpya, who knows that thread and him who pulls (it) within, he knows Brahman, he knows the worlds, he knows the Devas, he knows the Vedas, he knows the Bhûtas (creatures), he knows the Self, he knows everything." Thus did he (the Gandharva) say to them, and I know it. If thou, O Yâgñavalkya, without knowing that string and the puller within, drivest away those Brahma-cows (the cows offered as a prize to him who best knows Brahman), thy head will fall off.'
Yâgñavalkya said: 'O Gautama, I believe I know that thread and the puller within.'
The other said: 'Anybody may say, I know, I know. Tell what thou knowest.'