Śatapatha Brāhmaṇa ("of a hundred paths") 1.5.3.15

1.5.3.15

sa vai vyattu vetviti yajati | ajāmitāyai jāmi ha kuryādyadvyantuvyntviti vaiva yajedvetuvettviti vā vyantviti vai yoṣā vetviti vṛṣā mithunamevaitatprajananaṃ kriyate tasmādvyantu vetviti yajati

In order to avoid sameness he prays (alternately) with 'may they accept!' and 'may he (or it) accept!' for he would commit (the fault) of repetition, if he were to pray with 'may they accept!' each time, or with 'may he accept!' each time. By 'may they accept!' doubtless, females (are implied); and by 'may he accept!' a male (is implied): thereby a productive union is effected, and for this reason he prays (alternately) with 'may they accept!' and 'may he (or it) accept!'