Bṛhadāraṇyaka Upaniṣad ("Great Forest Teaching") 3.4.2

3.4.2

sa hovācoṣastaścākrāyaṇo yathā vibrūyādasau gaurasāvaśva ityevamevaitadvyapadiṣṭaṃ bhavati yadeva sākṣādaparokṣādbrahma ya ātmā sarvāntarastaṃ me vyācakṣvetyeṣa ta ātmā sarvāntaraḥ katamo yājñavalkya sarvāntaraḥ | na dṛṣṭerdraṣṭāraṃ paśyerna śruteḥ śrotāram̐ śṛṇuyā na matermantāraṃ manvīthā na vijñātervijñātāraṃ vijānīyāḥ | eṣa ta ātmā sarvāntaro'to'nyadārtaṃ tato hoṣastaścākrāyaṇa upararāma

Ushasta Kâkrâyana said: 'As one might say, this is a cow, this is a horse, thus has this been explained by thee. Tell me the Brahman which is visible, not invisible, the Self, who is within all.'
Yâgñavalkya replied: 'This, thy Self, who is within all.'
'Which Self, O Yâgñavalkya, is within all?'
Yâgñavalkya replied: 'Thou couldst not see the (true) seer of sight, thou couldst not hear the (true) hearer of hearing, nor perceive the perceiver of perception, nor know the knower of knowledge. This is thy Self, who is within all. Everything also is of evil.' After that Ushasta Kâkrâyana held his peace.

3.4.2.p1

iti tṛtīyādhyāye caturthaṃ brāhmaṇam