Śatapatha Brāhmaṇa ("of a hundred paths") 11.5.4.13

11.5.4.13

tāṃ haitāmeke sāvitrīmanuṣṭubhamanvāhurvāgvā anuṣṭuptadasminvācaṃ dadhma iti na tathā kuryādyo hainaṃ tatra brūyādā nvā ayamasya vācamadita mūko bhaviṣyatītīśvaro ha tathaiva syāttasmādetāṃ gāyatrīmeva sāvitrīmanubrūyāt

Now some teach an Anushtubh Sâvitrî, saying, 'The Anushtubh is speech: we thus lay speech into him.' But let him not do so; for if, in that case, any one were to say of him, 'Surely, this (student) has taken away his (the teacher's) speech: he will become dumb;' then that would indeed be likely to come to pass: let him therefore teach him that Gâyatrî Sâvitrî.