Shatapatha Brahmana 14.3.1.3

14.3.1.3

atha yanmukhadaghne uparīva vai tadyanmukhadaghnamuparīva tadyadasau lokastadyāmuṃ lokaṃ śugāviśattayaivainametacucā samardhayati kṛtsnaṃ karoti

And as to why (it is held) on a level with the mouth,--well, what is level with the mouth is, as it were, above; and above, as it were, is yonder (heavenly) world: thus he thereby supplies and completes it (the Pravargya) with that heat which had entered yonder world.