Śatapatha Brāhmaṇa ("of a hundred paths") 13.2.6.1

13.2.6.1

yuñjanti bradhnamaruṣaṃ carantamiti asau vā ādityo bradhno'ruṣo'mumevāsmā ādityaṃ yunakti svargasya lokasya samaṣṭyai

[He puts the horse to the chariot with Vâg. S. XXIII, 5], 'They harness the ruddy bay, moving (round the moveless: the lights shine in the heavens);'--the ruddy bay, doubtless, is yonder sun: it is yonder sun he harnesses for him, for the gaining of the heavenly world.