Śatapatha Brāhmaṇa ("of a hundred paths") 11.2.6.3

11.2.6.3

anūkamevāsya sāmidhenyaḥ tasmāttā brūyātsaṃtanvanniva me'nubrūhīti saṃtatamiva hīdamanūkam manaścaivāsya vākcāghārau sarasvāṃśca sarasvatī ca sa vidyānmanaścaiva me vākcāghārau sarasvāṃśca sarasvatī ceti

The kindling-verses, indeed, are its spine: let him therefore say (to the Hotri.) regarding them, 'Recite for me, making them, as it were, continuous;' for continuous, as it were, is this spinal column. And the two libations of ghee are its mind and speech, Sarasvat and Sarasvatî: let him know that the two libations of ghee are his mind and speech, Sarasvat and Sarasvatî.