Śatapatha Brāhmaṇa ("of a hundred paths") 3.2.1.10

3.2.1.10

atha mekhalām pariharate | aṅgiraso ha vai dīkṣitānabalyamavindatte nānyadvratādaśanamavākalpayaṃsta etāmūrjamapaśyantsamāptiṃ tām madhyata ātmana ūrjamadadhata samāptiṃ tayā samāpnuvaṃstatho evaiṣa etām madhyata ātmana ūrjaṃdhatte samāptiṃ tayā samāpnoti

He then girds himself with the zone. For once upon a time when the Aṅgiras were consecrated, they were seized with weakness, for they had prepared no other food but fast-milk. They then perceived this (source of) strength (viz. the zone), and this (source of) strength they put in (or round) the middle of their body as a (means of attaining) completion: and thereby they attained completion. And so does he now put that (source of) strength in the middle of his body and thereby attain completion.