Śatapatha Brāhmaṇa ("of a hundred paths") 9.3.1.17

9.3.1.17

tadāhuḥ | kathamasyaite puro 'nuvākyavanto yājyavanto vaṣaṭkṛte srucā hutā bhavantītyeteṣāṃ vai saptapadānām mārutānāṃ yāni trīṇi prathamāni padāni sā tripadā gāyatrī puro 'nuvākyātha yāni catvāryuttamāni sā catuṣpadā triṣṭubyājyedameva kapucalamayaṃ daṇḍaḥ svāhākāro vaṣaṭkāra evamu hāsyaite puro 'nuvākyavanto yājyavanto vaṣaṭkṛte srucā hutā bhavanti

As to this they say, 'How do these (Mâruta oblations) also come to be offered for him by the offering-ladle, at the Vashat-call, and with invitatory and offering formulas?' Well, the first three feet of those seven-footed Mâruta (verses), being a three-footed Gâyatrî, are the invitatory formula, and the last four, being a four-footed Trishtubh, are the offering-formula. The one is the bowl, and the other the handle (of the offering-ladle), and the Svâhâ-call is the Vashat-call: in this way, then, these (Mâruta oblations) also become offered for him by the offering-ladle, at the Vashat-call, and with invitatory and offering-formulas.