Shatapatha Brahmana 5.3.4.25

5.3.4.25

atha yatsārasvatīṣu na juhoti | vāgvai sarasvatī vajra ājyaṃ nedvajreṇājyena vācaṃ hinasānīti tasmātsārasvatīṣu na juhoti

And as to why he does not offer on the (water) from the Sarasvatî,--Sarasvatî, to be sure, is (the goddess of) Speech, and the ghee is a thunderbolt: 'Lest I should injure (the goddess of) Speech! thus (he thinks, and) therefore he does not offer on the water from the Sarasvatî.