Śatapatha Brāhmaṇa ("of a hundred paths") 1.7.2.21

1.7.2.21

sa vai vaugiti karoti | vāgvai vaṣaṭkāro vāgreto reta evaitatsiñcati ṣaḍityṛtavo vai ṣaṭtadṛtuṣvevaitadretaḥ sicyate tadṛtavo retaḥ siktamimāḥ prajāḥ prajanayanti tasmādevaṃ vaṣaṭkaroti

He pronounces (the syllable) vauk; for, assuredly, the vashat-call is speech; and speech means seed: hence he thereby casts seed. 'Shat' (he pronounces), because there are six seasons: he thereby casts that seed into the seasons, and the seasons cause that seed so cast to spring up here as creatures. This is the reason why he pronounces the vashat.