Shatapatha Brahmana 14.3.1.5

14.3.1.5

atha dvitīyamujjvalayya nābhidaghne dhārayamāṇo juhoti madhyamiva vai tadyannābhidaghnaṃ madhyamivāntarikṣalokastadyāntarikṣalokaṃ śugāviśattayaivainametacucā samardhayati kṛtsnaṃ karoti

Then, having kindled the second (bundle), he offers (thereon) whilst holding it navel-high; for in the middle, as it were, is what is navel-high, and in the middle, as it were, is the air-world: thus he thereby supplies and completes it with that heat which had entered the air-world.