Jaiminīya Upaniṣad Brāhmaṇa 2.6.8

2.6.8

sa yadi brūyān nava ma āgāyeti prāṇa udgītha ity eva vidvān nava manasā dhyāyet
sapta śīrṣaṇyāḥ prāṇā dvāv avāñcau
nava haivāsyājāyante /

If he should say: “Chant for me for nine”, knowing that breath is the udgītha, he should mentally focus on nine.
Seven are the breaths in the head, two are downward.
Nine indeed are born to him.