Śatapatha Brāhmaṇa ("of a hundred paths") 1.6.4.18

1.6.4.18

tadvā eṣa evendraḥ | ya eṣa tapatyathaiṣa eva vṛtro yaccandramāḥ mo 'syaiṣa bhrātṛvyajanmeva tasmādyadyapi purā vidūramivodito 'thainametāṃ rātrimupaiva nyāplavate so 'sya vyāttamāpadyate

Now the one that burns there (viz. the sun) is, assuredly, no other than Indra, and that moon is no other than Vritra. But the former is of a nature hostile to the latter, and for this reason, though this one (the moon, Vritra) had previously (to the night of new moon) risen at a great distance from him (the sun, Indra), he now swims towards him and enters into his open mouth.