Bṛhadāraṇyaka Upaniṣad ("Great Forest Teaching") 4.1.2

4.1.2

yatte kaścidabravīttacchṛṇavāmetyabravīnme jitvā śailinirvāgvai brahmeti yathā mātṛmānpitṛmānācāryavānbrūyāttathā tacchailinirabravīdvāgvai brahmetyavadato hi kim̐ syādityabravīttu te tasyāyatanaṃ pratiṣṭhāṃ na me'bravīdityekapādvā etatsamrāḍiti sa vai no brūhi yājñavalkya | vāgevāyatanamākāśaḥ pratiṣṭhā prajñetyenadupāsīta | kā prajñatā yājñavalkya vāgeva samrāḍiti hovāca | vācā vai samrāḍvandhuḥ prajñāyata gvedo yajurvedaḥ sāmavedo'tharvāṅgirasa itihāsaḥ purāṇaṃvidyā upaniṣadaḥ ślokāḥ sūtrā-ṇyanuvyākhyānāni vyākhyānāniṣṭam̐ hutamāśitaṃpāyitamayaṃ ca lokaḥ paraśca lokaḥ sarvāṇi ca bhūtāni vācaiva samrāprajñāyante vāgvai samrāṭparamaṃ brahma nainaṃ vāgjahāti sarvāṇyenaṃ bhūtānyabhikṣarantidevo bhūtvā devānapyeti ya evaṃ vidvānetadupāste | hastyṛṣabham̐ sahasraṃ dadāmīti hovāca janako vaidehaḥ | sa hovāca yājñavalkyaḥ pitā me'manyata nānanuśiṣya hareteti

'Let us hear what anybody may have told you.'
Ganaka Vaideha replied: 'Gitvan Sailini told me that speech (vâk) is Brahman.'
Yâgñavalkya said: 'As one who had (the benefit of a good) father, mother, and teacher might tell, so did Sailini tell you, that speech is Brahman; for what is the use of a dumb person? But did he tell you the body (âyatana) and the resting-place (pratishthâ) of that Brahman?'
Ganaka Vaideha said: 'He did not tell me.'
Yâgñavalkya said: 'Your Majesty, this (Brahman) stands on one leg only.'
Ganaka Vaideha said: 'Then tell me, Yâgñavalkya.'
Yâgñavalkya said: 'The tongue is its body, ether its place, and one should worship it as knowledge.'
Ganaka Vaideha said: 'What is the nature of that knowledge?'
Yâgñavalkya replied: 'Your Majesty, speech itself (is knowledge). For through speech, Your Majesty, a friend is known (to be a friend), and likewise the Rig-Veda, Yagur-veda, Sâma-veda, the Atharvâṅgirasas, the Itihâsa (tradition), Purâna-vidyâ (knowledge of the past), the Upanishads, Slokas (verses), Sûtras (rules), Anuvyâkhyânas and Vyâkhyânas (commentaries, &c.); what is sacrificed, what is poured out, what is (to be) eaten and drunk, this world and the other world, and all creatures. By speech alone, Your Majesty, Brahman is known, speech indeed, O King, is the Highest Brahman. Speech does not desert him who worships that (Brahman) with such knowledge, all creatures approach him, and having become a god, he goes to the gods.'
Ganaka Vaideha said: 'I shall give you (for this) a thousand cows with a bull as big as an elephant.'
Yâgñavalkya said: 'My father was of opinion that one should not accept a reward without having fully instructed a pupil.'