Śatapatha Brāhmaṇa ("of a hundred paths") 1.5.3.13

1.5.3.13

atha svāhāsvāheti yajati | anto vai yajñasya svāhākāro 'nta ṛtūnāṃ hemanto vasantāddhi parārddhyo 'ntenaiva tadantaṃ devā avṛñjatāntenāntātsapatnānantarāyannanteno evaiṣa etadantaṃ vṛṅkte 'ntenāntātsapatnānantareti tasmātsvāheti yajati

He then pronounces the offering-prayer with 'Svâhâ! Svâhâ!' The Svâhâ-call, namely, marks the end of the sacrifice, and the end of the year is the winter, since the winter is on the other (remoter) side of the spring. By the end (of the sacrifice) the gods, at that time, appropriated the end (of the year); by the end they deprived their rivals of the end; and by the end this one also now appropriates the end; by the end he deprives his rivals of the end: this is why he pronounces the offering-prayers with 'Svâhâ! Svâhâ!'