Śatapatha Brāhmaṇa ("of a hundred paths") 9.2.3.24

9.2.3.24

athāgnimārohanti | kramadhvamagninā nākamiti svargo vai loko nākaḥ kramadhvamanenāgninaitaṃ svargaṃ lokamityetadukhyaṃ hasteṣu bibhrata ityukhyaṃ hyeta etaṃ hasteṣu bibhrati divaspṛṣṭhaṃ svargatvā miśrā devebhirādhvamiti divaspṛṣṭhaṃ svargaṃ lokaṃ gatvā miśrā devebhirādhvamityetat

They then mount the fire-altar, with (Vâg. S. XVII, 65-69), 'By Agni ascend ye to the firmament!'--the firmament, doubtless, is the heavenly world: thus, 'by means of this Agni (fire-altar) ascend ye to that heavenly world!'--'holding the Ukhya in your hands,'--for the Ukhya (the fire in the pan) they do hold in their hands;--'having gone to the back of the sky, to heaven, keep ye mingling with the gods!'--that is, 'having gone to the back of the sky, to the heavenly world, keep ye mingling with the gods!'