Śatapatha Brāhmaṇa ("of a hundred paths") 3.1.4.16

3.1.4.16

atha yām pañcamīṃ srucā juhoti | sā haiva pratyakṣaṃ yajño 'nuṣṭubhā hi tāṃ juhoti vāgghyanuṣṭubvāgghi yajñaḥ

But the fifth libation which he makes with the offering-spoon (sruk), doubtless is the veritable sacrifice; for he offers it with an anushtubh (verse), and the anushtubh is speech and so is the sacrifice.