Śatapatha Brāhmaṇa ("of a hundred paths") 13.2.2.16

13.2.2.16

hiraṇmayo'śvasya śāso bhavati lohamayāḥ paryaṅgyāṇāmāyasā itareṣāṃ jyotirvai hiraṇyaṃ rāṣṭramaśvamedho jyotireva tadrāṣṭre dadhātyatho hiraṇyajyotiṣaiva yajamānaḥ svargaṃ lokametyatho anūkāśameva taṃ kurute svargasya lokasya samaṣṭyai

The slaughtering-knife of the horse is made of gold, those of the 'paryaṅgyas' of copper, and those of the others of iron; for gold is (shining) light, and the Asvamedha is the royal office: he thus bestows light upon the royal office. And by means of the golden light (or, by the light of the gold), the Sacrificer also goes to the heavenly world; and he, moreover, makes it a gleam of light shining after him, for him to reach the heavenly world.