Śatapatha Brāhmaṇa ("of a hundred paths") 6.7.1.2

6.7.1.2

tadyattatsatyam | asau sa ādityaḥ sa hiraṇmayo bhavati jyotirvai hiraṇyaṃ jyotireṣo 'mṛtaṃ hiraṇyamamṛtameṣa parimaṇḍalo bhavati parimaṇḍalo hyeṣa ekaviṃśatinirbādha ekaviṃśo hyeṣa bahiṣṭānnirbādham bibharti raśmayo vā etasya nirbādhā bāhyata u vā etasya raśmayaḥ

Now that truth is the same as yonder sun. It is a gold (plate), for gold is light, and he (the sun) is the light; gold is immortality, and he is immortality. It (the plate) is round, for he (the sun) is round. It has twenty-one knobs, for he is the twenty-first. He wears it with the knobs outside, for the knobs are his (the sun's) rays, and his rays arc outside.