Jaiminīya Upaniṣad Brāhmaṇa 3.34.2

3.34.2

tad yatrāda āha somaḥ pavata iti vopāvartadhvam iti vā tat sahaiva vācā manasā prāṇena svareṇa hiṅkurvanti
tad hiṅkāreṇa mithunaṃ kriyate /

So when yonder [the priest] says, "Soma purifies itself," or "Turn toward [us]," then together with speech, mind, breath, and sound they utter the hiṅkāra. Thus by the hiṅkāra a pair is formed.