Śatapatha Brāhmaṇa ("of a hundred paths") 9.4.4.3

9.4.4.3

sa madhyamam paridhimupaspṛśya | etadyajurjapatyagniṃ yunajmi śavasā ghṛteneti balaṃ vai śavo 'gniṃ yunajmi balena ca ghṛtena cetyetaddivyaṃ suparṇaṃ vayasā bṛhantamiti divyo vā eṣa suparṇo vayaso bṛhandhūmena tena vayaṃ gamema bradhnasya viṣṭapaṃ svargaṃ lokaṃ rohanto 'dhi nākamuttamamityetat

Having touched the middle enclosing-stick, he mutters this formula (Vâg. S. XVIII, 51), 'The Fire I yoke with might, with ghee!'--might means strength: thus, 'The fire I yoke (furnish) with strength, with ghee;'--'the heavenly bird, great in vigour,'--for that (fire-altar) is indeed a heavenly bird, and great in vigour, in smoke;--'Thereby we will go to the region of the bay, rising unto the light, beyond the highest firmament!'--The firmament, doubtless, is the heavenly world: thus, 'Thereby we will go to the region, of the bay (horse, the sun) mounting up to the heavenly world, beyond the highest firmament.'