Kauṣītaki Upaniṣad 3.3.17

3.3.17

yatraitat puruṣaḥ suptaḥ svapnaṃ na kañcana paśyaty athāsmin prāṇa evaikadhā bhavanti /

When a person is so asleep that he sees no dream whatever, then he becomes unitary in this breathing spirit. Then speech together with all names goes to it; the eye together with all forms goes to it; the ear together with all sounds goes to it; the mind together with all thoughts goes to it.