Śatapatha Brāhmaṇa ("of a hundred paths") 1.4.1.2

1.4.1.2

yadveva hiṃkaroti | prāṇo vai hiṃkāraḥ prāṇo hi vai hiṃkārastasmādapigṛhya nāsike na hiṃkartuṃ śaknoti vācā vā ṛcamanvāha vākca vai prāṇaśca mithunaṃ tadetatpurastānmithunam prajananaṃ kriyate sāmidhenīnāṃ tasmādvai hiṃkṛtyānvāha

And (another reason) why he utters 'Hiṅ!' is this. The word 'Hiṅ' means breath, for the word Hiṅ' does indeed mean breath: he cannot therefore pronounce the word 'Hiṅ,' when he closes his nostrils. The rik (verse) he recites with his voice. Now, voice and breath are a pair, so that a productive union of the sâmidhenîs is thereby effected at the outset: for this reason he recites, after uttering 'Hiṅ!'