Śatapatha Brāhmaṇa ("of a hundred paths") 1.7.2.14

1.7.2.14

tadyathā yonau retaḥ siñcet | evaṃ tadatha yatpurā vaṣaṭkārājjuhuyādyathā yonau retaḥ siktaṃ tadamuyā syādevaṃ tattasmādu saha vaiva vaṣaṭkāreṇa juhuyādvaṣaṭkṛte vā

As seed is poured into the womb, so here. If, on the other hand, he were to make the offering before the vashat, it would be lost, as would be the seed poured not into the womb: for this reason also let him make the offering either simultaneously with the vashat or after it has been pronounced.