Śatapatha Brāhmaṇa ("of a hundred paths") 2.5.2.31

2.5.2.31

athādhvaryurevāhāgnaye 'nubrūhīti | āgneyamājyabhāgaṃ tā ubhāveva caturājyasyāvadāyātikrāmato 'tikramyāśrāvyādhvaryurevāhāgniṃ yajeti tā ubhāveva vaṣaṭkṛte juhutaḥ

Thereupon the Adhvaryu says (to the Hotri), 'Pronounce the invitatory prayer to Agni!' referring to Agni's butter-portion. Both (the Adhvaryu and Pratiprasthâtri) having taken four 'cuttings' of butter, they step across (to the north side of their respective fires). Having stepped across and called for the 'Sraushat,' the Adhvaryu says (to the Hotri), 'Pronounce the offering-formula to Agni!' After the 'Vashat' has been uttered, they both pour out the oblation.