Śatapatha Brāhmaṇa ("of a hundred paths") 1.7.2.12

1.7.2.12

so 'nuvākyāmanūcya | yājyāmanudrutya paścādvaṣaṭkaroti paścādvai parītya vṛṣā yoṣāmadhidravati tadene ubhe purastātkṛtvā vṛṣṇā vaṣaṭkāreṇādhidrāvayati tasmādu saha vaiva vaṣaṭkāreṇa juhuyādvaṣaṭkṛte vā

Having recited the invitatory prayer and pronounced the offering-prayer, he afterwards (paskât) utters the vashat formula; for from behind (paskât) the male approaches the female: hence, after placing those two in front, he causes them to be approached by that male, the vashat. For the same reason let him make the offering either simultaneously with the vashat or (immediately) after the vashat has been pronounced.