Śatapatha Brāhmaṇa ("of a hundred paths") 1.1.3.2

1.1.3.2

te vai dve bhavataḥ | ayaṃ vai pavitraṃ yo 'yam pavate so 'yameka ivaiva pavate so 'yam puruṣe 'ntaḥ praviṣṭaḥ prāṅca pratyaṅca tāvimau prāṇodānau tadetasyaivānu mātrāṃ tasmāddve bhavataḥ

Two there are of them: for means of cleansing (pavitra) is this (wind) which here ventilates (pavate); and this, it is true, ventilates as one only; but on entering into man, it becomes a forward and a backward one, and they are these two, to wit, the prâna (breathing out) and the udâna (breathing up or in). And as this (clarifying process) takes place in accordance with the measure of that (process of breathing), therefore there are two (strainers).