Śatapatha Brāhmaṇa ("of a hundred paths") 7.4.1.39

7.4.1.39

yadveva srucā upadadhāti | prajāpatervisrastasyāgnisteja ādāya dakṣiṇākarṣatso 'trodaramadyatkṛṣṭvodaramattasmātkārṣmaryo 'thāsyendra oja ādāyodaṅṅadakrāmatsa udumbaro 'bhavat

And, again, why he lays down two offering-spoons. When Pragâpati was relaxed, Agni took his (Pragâpati's) fiery spirit, and carried it off to the south, and there stopped; and because after carrying (karsh) it off he stopped (ud-ram), therefore the Kârshmarya (sprang up). And Indra took his (Pragâpati's) vigour and went away to the north: it became the Udumbara tree.