Jaiminīya Upaniṣad Brāhmaṇa 2.6.6

2.6.6

sa yadi brūyāt ṣaṇ ma āgāyeti prāṇa udgītha ity eva vidvān ṣaṇ manasā dhyāyet
ṣaḍ ḍhi prāṇo 'pāno vyānas samāno 'vāna udānaḥ
ṣaḍ ḍhaivāsyājāyante /

If he should say: “Chant for me for six”, knowing that breath is the udgītha, he should mentally focus on six.
For breath, out-breath, diffused breath, equalized breath, downward breath, and upward breath are six.
Six indeed are born to him.