Śatapatha Brāhmaṇa ("of a hundred paths") 3.7.2.8

3.7.2.8

atha patnībhyaḥ patnīyūpamucrayanti | sarvatvāya nveva patnīyūpa ucrāyate tattvāṣṭram paśumālabhate tvaṣṭā vai siktaṃ reto vikaroti tadeṣa evaitatsiktaṃ reto vikaroti muṣkaro bhavatyeṣa vai prajanayitā yanmuṣkarastasmānmuṣkaro bhavati taṃ na saṃsthāpayetparyagnikṛtamevotsṛjetsa yatsaṃsthāpayetprajāyai hāntamiyāttatprajāmutsṛjati tasmānna saṃsthāpayetparyagnikṛtamevotsṛjet

Thereupon they set up the wife-stake for the wives. It is for the sake of completeness, forsooth, that the wife-stake is set up: there they seize (and bind) the victim for Tvashtri, for Tvashtri fashions the cast seed, and hence he fashions the seed now cast. It (the victim to Tvashtri) is an animal with testicles, for such a one is a begetter. Let him not slay that one, but let him set it free after fire has been carried round it. Were he to slay it, there would assuredly be an end to offspring, but in this way he sets free the offspring. Therefore let him not slay it, but let him set it free after fire has been carried round it.