Shatapatha Brahmana 9.1.2.21

9.1.2.21

yadvevainaṃ vikarṣati | etadvai yatraitam prāṇā ṛṣayo 'gre 'gniṃ samaskurvaṃstamadbhiravokṣaṃstā āpaḥ samaskandaṃste maṇḍūkā abhavan

And, again, as to why he draws them across. Now, in the beginning, when the Rishis, the vital airs, made up that Agni, they sprinkled him with water: that water dripped off and became the frogs.