Varahashrautasutra 3.2.2.23

3.2.2.23

o śrāvayeti caturakṣaram astu śrauṣaḍiti caturakṣam | yajeti dvyakṣaram | ye yajāmaha iti pañcākṣaram | dvyakṣaro vaṣaṭkāraḥ | eṣa vai hotaḥ prajāpatiḥ saptadaśaḥ

"Om, call to attention!" has four syllables; "May it be heard!" has four syllables; "Sacrifice!" has two syllables; "We who sacrifice..." has five syllables; the vaṣaṭ-call has two syllables: this verily, O hotṛ, is the seventeenfold Prajāpati.