Śatapatha Brāhmaṇa ("of a hundred paths") 8.6.3.18

8.6.3.18

yena ṛṣayastapasā sattramāyanniti | amūnetadṛṣīnāhendhānā agniṃ svarābharanta itīndhānā agniṃ svargaṃ lokamāharanta ityetattasminnahaṃ nidadhe nāke agnimiti svargo vai loko nāko yamāhurmanava stīrṇabarhiṣamiti ye vidvāṃsaste manava stīrṇabarhiṣamiti sarvadā haiva sa stīrṇabarhiḥ

[He lays down the first brick, with, Vâg. S. XV, 49], 'With what fervour the Rishis entered upon the sacrificial session,'--he thereby means those Rishis (the vital airs);--'kindling the fire and gaining the light,'--that is, 'kindling the fire, and gaining the heavenly world;'--'upon that firmament I place the Fire,'--the firmament, doubtless, is the heavenly world;--'whom thinkers call the straw-spreader,'--the thinkers (or men) are they who are wise; and 'straw-spreader' he says, because he (Agni) has ever the (sacrificial) straw spread for him.