Śatapatha Brāhmaṇa ("of a hundred paths") 10.6.5.2

10.6.5.2

āpo vā arkaḥ tadyadapāṃ śara āsīttatsamahanyata sā pṛthivyabhavattasyāmaśrāmyattasya śrāntasya taptasya tejo raso niravartatāgniḥ

The Arka, doubtless, is the waters; and the cream (froth) which was on the waters was compacted, and became this earth. Thereon he wearied himself, and the glow and essence (sweat) of him thus wearied and heated developed into Fire.