Śatapatha Brāhmaṇa ("of a hundred paths") 7.3.1.28

7.3.1.28

tā uttaravedau nivapati | yonirvā uttaravediryonau tadretaḥ siñcati yadvai yonau retaḥ sicyate tatprajaniṣṇu bhavati tābhiḥ sarvamātmānam pracādayati sarvasmiṃstadātmanreto dadhāti tasmātsarvasmādevātmano retaḥ sambhavati

He throws it on the Uttara-vedi;--the Uttaravedi is the womb: he thus infuses seed into the womb; and the seed which is infused into the womb becomes generative. He covers the whole body (of the altar) with that (sand): he thus puts seed into the whole body; whence the seed is produced from the whole body.