Praśna Upaniṣad ("Six Questions") 3.5

3.5

pāyūpasthe'pānaṃ cakṣuḥśrotre mukhanāsikābhyāṃ prāṇaḥ svayaṃ prātiṣṭhate madhye tu
samānaḥ
eṣa hyetaddhutamannaṃ samaṃ nayati tasmādetāḥ saptārciṣo bhavanti

5. The out-breath (apāna) is in the organs of excretion and generation. The life-breath (prāṇa) as such (svayam) establishes itself in the eye and ear, together with the mouth and nose. While in the middle is the equalizing breath (samāna),

for it is this [breath] that equalizes [in distribution] whatever has been offered as food. From this arise the seven flames.