Shatapatha Brahmana 9.1.1.13

9.1.1.13

atha mukhadaghna | uparīva vai tadyanmukhadaghnamuparīva tadyadasau lokastadye 'muṃ lokaṃ rudrāḥ prāviśaṃstāṃstatprīṇāti svāhākāreṇānnaṃ vai svāhākāro 'nnenaivainānetatprīṇāti

Then on the one reaching up to the mouth,--what reaches up to the mouth is, as it were, above, and above, as it were, is yonder world: he thus gratifies those Rudras who entered yonder world. [He does so] with Svâhâ,--the Svâhâ is food: with food he thus gratifies them.