Bṛhadāraṇyaka Upaniṣad ("Great Forest Teaching") 1.5.3

1.5.3

trīṇyātmane'kuruteti mano vācaṃ prāṇaṃ tānyātmane'kurutānyatramanā abhūvaṃ nādarśamanyatramanā abhūvaṃ nāśrauṣamiti manasā hyeva paśyati manasā śṛṇoti | kāmaḥ saṃkalpo vicikitsā śraddhāśraddhā dhṛtiradhṛtirhrīrdhīrbhīrityetatsarvaṃ mana eva tasmādapi pṛṣṭhata upaspṛṣṭo manasā vijānāti yaḥ kaśca śabdo vāgeva saiṣā hyantamāyattaiṣā hi na prāṇo'pāno vyāna udānaḥ samāno'na ityetatsarvaṃ prāṇa evaitanmayo vā ayamātmā vāṅmayo manomayaḥ prāṇamayaḥ

When it is said, that 'he made three for himself,' that means that he made mind, speech, and breath for himself. As people say, 'My mind was elsewhere, I did not see; my mind was elsewhere, I did not hear,' it is clear that a man sees with his mind and hears with his mind. Desire, representation, doubt, faith, want of faith, memory, forgetfulness, shame, reflexion, fear, all this is mind. Therefore even if a man is touched on the back, he knows it through the mind.
Whatever sound there is, that is speech. Speech indeed is intended for an end or object, it is nothing by itself.
The up-breathing, the down-breathing, the back-breathing, the out-breathing, the on-breathing, all that is breathing is breath (prâna) only. Verily that Self consists of it; that Self consists of speech, mind, and breath.