Shatapatha Brahmana 10.3.5.15

10.3.5.15

tadetadyajurupāṃśvaniruktam prāṇo vai yajurupāṃśvāyatano vai prāṇastadya enaṃ nirbruvantam brūyādaniruktāṃ devatāṃ niravocatprāṇa enaṃ hāsyatīti tathā haiva syāt

This Yagus is silent, indistinct; for the Yagus is the breath, and the breath is of silent (secret) abode; and if any one were to say of that (Adhvaryu) who pronounces (the Yagus) distinctly, 'He has uttered distinctly the indistinct deity: his breath shall fail him!' then that would, indeed, come to pass.