Kauṣītaki Upaniṣad 2.1.5

2.1.5

vāk pariveṣṭrī [sa yo ha vā etasya prāṇasya brahmaṇo mano dūtam veda dūtavān bhavati /

Of this same breathing spirit as Brahma, verily, indeed, the mind (_manas_) is the messenger; the eye, the watchman; the ear, the announcer; speech, the handmaid.

He who, verily, indeed, knows the mind as the messenger of this breathing spirit, [i. e.] of Brahma, becomes possessed of a messenger; he who knows the eye as the watchman, becomes possessed of a watchman; he who knows the ear as the announcer, becomes possessed of an announcer; he who knows speech as the handmaid, becomes possessed of a handmaid.