Bṛhadāraṇyaka Upaniṣad ("Great Forest Teaching") 4.1.6

4.1.6

yadeva tekaścidabravīttacchṛṇavāmetyabravīnme satyakāmo jābālo mano vai brahmeti yathā mātṛmānpitṛmānācāryavānbrūyāttathā tajjābālo'bravīnmano vai brahmetyamana so hi kim̐ syādityabravīttu te tasyāyatanaṃ pratiṣṭhāṃ na me'bravīdityekapādvā etatsamrāḍiti sa vai no brūhi yājñavalkya mana evāyatanamākāśaḥ pratiṣṭhānanda ityenadupāsīta kānandatā yājñavalkya mana eva samrāḍiti hovāca manasā vai samrāṭstriyamabhihāryate tasyāṃ pratirūpaḥ putro jāyate sa ānandomano vai samrāṭ paramaṃ brahma nainaṃ mano jahāti sarvaṇyenaṃ bhūtānyabhikṣaranti devo bhūtvā devānapyeti ya evaṃ vidvānetadupāste hastyṛṣabham̐ sahasraṃ dadāmīti hovāca janako vaidehaḥ sa hovāca yājñavalkyaḥ pitā me'manyata nānanuśiṣya haretetiṃ

Yâgñavalkya said: 'Let us hear what anybody may have told you.'
Ganaka Vaideha replied: 'Satyakâma Gâbâla told me that mind (manas) is Brahman.'
Yâgñavalkya said: 'As one who had (the benefit of a good) father, mother, and teacher might tell, so did Satyakâma Gâbâla tell you that mind is Brahman; for what is the use of a person without mind? But did he tell you the body and the resting-place 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: 'Mind itself is its body, ether its place, and we should worship it as bliss.'
Ganaka Vaideha said: 'What is the nature of bliss?'
Yâgñavalkya replied: 'Your Majesty, mind itself; for with the mind does a man desire a woman, and a like son is born of her, and he is bliss. Mind indeed, O King, is the Highest Brahman. Mind 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.'