Shatapatha Brahmana 5.5.3.7

5.5.3.7

āsandyā upānahā upamuñcate | upānaḍbhyāmadhi yadasya yānam bhavati ratho vā kiṃcidvā sarvaṃ vā eṣa idamuparyupari bhavatyarvāgevāsmādidaṃ sarvam bhavati yo rājasūyena yajate tasmādasyaiṣaiva vratacaryā bhavati yāvajjīvaṃ nāsyām pratitiṣṭhati

From the throne-seat he slips into the shoes; and on shoes (he stands), whatever his vehicle may be, whether a chariot or anything else. For verily he who performs the Râgasûya is high above everything here, and everything here is beneath him;--therefore this is for him a religious observance: as long as he lives he does not stand on the earth (with bare feet).