Śatapatha Brāhmaṇa ("of a hundred paths") 2.5.1.4

2.5.1.4

tasmādetadṛṣiṇābhyanūktam | prajā ha tisro atyāyamīyuriti tadyāḥ parābhūtāstā evaitadabhyanūktaṃ nyanyā arkamabhito viviśra ityagnirvā arkastadyā imāḥ prajā aparābhūtāstā agnimabhito niviṣṭāstā evaitadabhyanūktam

Hence it has been said by the Rishi,.--'Three generations have passed beyond,'--this is said regarding those that passed away;--'Others settled down around the light (arka, the sun)'--the light doubtless is the fire: those creatures which did not pass away, settled down around the fire; it is with regard to them that this is said.