Shatapatha Brahmana 1.9.1.29

1.9.1.29

athānayetyupaspṛśati | amānuṣa iva vā etadbhavati yadārtvijye pravṛta iyaṃ vai pṛthivī pratiṣṭhā tadasyāmevaitatpratiṣṭhāyām pratiṣṭhati tadu khalu punarmānuṣo bhavati tasmādanayetyupaspṛśati

He then touches (the earth) thus with this (finger). Non-human, verily, he becomes at the time when he is chosen for the office of sacrificial priest; and, this earth being a safe standing-place, he thereby (viz. by touching the earth) stands on this safe standing-place; and he thereby also again becomes human: for this reason he thus touches (the earth) with this (finger).