Śatapatha Brāhmaṇa ("of a hundred paths") 1.4.1.5

1.4.1.5

yadveveti ca preti cānvāha | preti vai prāṇa etyudānaḥ prāṇodānāvevaitaddadhāti tasmādvā eti ca preti cānvāha 1.4.1[[.]]6 yadveveti ca preti cānvāha | preti vai retaḥ sicyata eti prajāyate preti paśavo vitiṣṭanta eti samāvartante sarvaṃ vā idameti ca preti ca tasmādvā eti ca preti cānvāha

And (another reason) why he recites with 'â' and 'pra,' is this. 'Pra (forth)' clearly means out-breathing, and 'â (hither)' means in-breathing: hence he thereby obtains out-breathing and in-breathing (for the sacrificer). For this reason he recites with 'â' and 'pra.'