Śatapatha Brāhmaṇa ("of a hundred paths") 6.2.2.6

6.2.2.6

athaitaṃ vāyave niyutvate | śuklaṃ tūparamālabhate prajāpatiḥ prajāḥ sṛṣṭvānuvyaikṣata tasyātyānandena retaḥ parāpatatso 'jaḥ śuklastūparo lapsudyabhavadraso vai reto yāvānu vai rasastāvānātmā tadyadetamālabhate tadevāgnerantam paryeti śuklo bhavati śuklaṃ hi retastūparo bhavati tūparaṃ hi reto vāyave bhavati prāṇo vai vāyurniyutvate bhavatyudāno vai niyutaḥ prāṇodānāvevāsminnetaddadhāti

He then slaughters for Vâyu Niyutvat (the wind, driving a team of horses) that white, bearded (he-goat). When Pragâpati had produced living beings, he looked about him, and from exceeding delight his seed fell: it became that white, hornless, bearded he-goat (aga, 'unborn'); for seed is life-sap, and as far as there is life-sap, so far extends the self. And when he slaughters that one, then indeed he reaches the end of Agni (the fire-altar). It is a white one, because seed is white. It is hornless, because seed is hornless. It belongs to Vâyu, because Vâyu (the wind) is the out-breathing; and to Niyutvat, because the teams (niyut) are the in-breathing: the out-breathing and in-breathing he thus lays into him.