Śatapatha Brāhmaṇa ("of a hundred paths") 11.2.7.27

11.2.7.27

prāṇā vai prayājāḥ apānā anuyājāstasmātprayājāḥ prāñco hūyante taddhi prāṇarūpam pratyañco'nuyājāstadapānarūpametā ha vai darśapūrṇamāsayorupasado yadanuyājāstasmātta upasadrūpeṇa pratyañco hūyante

The fore-offerings, indeed, are the out-breathings, and the after-offerings the off-breathings wherefore the fore-offerings are poured out in a forward direction, for that is the form of the out-breathing; and the after-offerings (are poured out) in a backward direction, for that is the form of the off-breathing. The after-offerings, indeed, are the Upasads of the Full and New-moon sacrifices, whence they are performed in a backward direction after the manner of the Upasads.