Śatapatha Brāhmaṇa ("of a hundred paths") 3.1.3.18

3.1.3.18

athainaṃ darbhapavitreṇa pāvayati | amedhyo vai puruṣo yadanṛtaṃ vadati tena pūtirantarato medhyā vai darbhā medhyo bhūtvā dīkṣā iti pavitraṃ vai darbhāḥ pavitrapūto dīkṣā iti tasmādenaṃ darbhapavitreṇa pāvayati

He then purifies him with a cleanser (pavitra, strainer) of sacred grass; for impure, indeed, is man:--he is foul within in that he speaks untruth;--and sacred grass is pure: 'Having become pure, I shall be consecrated,' thus he thinks;--and the stalks of sacred grass are a means of cleansing,-- 'Having become cleansed, I shall be consecrated,' thus he thinks; and therefore he purifies him with a cleanser of sacred grass.