Śatapatha Brāhmaṇa ("of a hundred paths") 2.5.2.33

2.5.2.33

tadyatkiṃ ca vācā kartavyam | adhvaryureva tatkaroti na pratiprasthātā tadyadadhvaryurevāśrāvayatīhaiva yatra vaṣaṭkriyate

Thus whatever has to be done by speech, that the Adhvaryu does, and not the Pratiprasthâtri. Now as to why the Adhvaryu alone calls for the 'Sraushat.' Here indeed when the 'Vashat' is pronounced,