Śatapatha Brāhmaṇa ("of a hundred paths") 3.2.1.15

3.2.1.15

atha nīvimudgūhate | somasya nīvirasītyadīkṣitasya vā asyaiṣāgre nīvirbhavatyathātra dīkṣitasya somasya tasmādāha somasya nīvirasīti

He then tucks up the end of his (nether) garment, with the text, 'Thou art Soma's tuck.' For heretofore it was the tuck of him, the unconsecrated; but now that he is consecrated, it is that of Soma: therefore he says, 'Thou art Soma's tuck.'