Jaiminīya Upaniṣad Brāhmaṇa 1.53.2

1.53.2

tayor yat sat tat sāma tan manas sa prāṇaḥ
atha yad asat sa rk sā vāk so 'pānaḥ /

Of the two, that which is the real is the sāman, it is the mind, it is the out-breath; and that which is the unreal is the ṛc, it is speech, it is the in-breath.