Shatapatha Brahmana 6.4.1.8

6.4.1.8

apām pṛṣṭhamasi yoniragneriti | apāṃ hyetatpṛṣṭhaṃ yonirhyetadagneḥ samudramabhitaḥ pinvamānamiti samudro hyetadabhitaḥ pinvate vardhamāno mahāṃ ā ca puṣkara iti vardhamāno mahīyasva puṣkara ityetaddivo mātrayā varimṇā pathasvetyanuvimārṣṭyasau vā āditya eṣo 'gnirno haitamanyo divo varimā yantumarhati dyaurbhūtvanaṃ yacetyevaitadāha

[Vâg. S. XI, 29] 'Thou art the waters’ back, Agni's womb,' for this is indeed the back of the waters, and the womb of Agni;--'around the swelling sea,'--for the sea indeed swells around it;--'thou, growing mighty upon the lotus,'--that is, 'growing, prosper thou on the lotus.'--'With the measure of the sky, extend thou in width!'--with this he strokes along it (so as to lie even on the skin); for that Agni is yonder sun; and him assuredly none other than the width of the sky can contain: 'having become the sky, contain him!' this is what he thereby says.