Kauṣītaki Upaniṣad 2.1.6

2.1.6

yaś cakṣur goptṛ goptṛmān bhavati /

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.