Śatapatha Brāhmaṇa ("of a hundred paths") 5.2.1.8

5.2.1.8

atha neṣṭā patnīmudāneṣyan | kauśaṃ vāsaḥ paridhāpayati kauśaṃ vā caṇḍātakamantaraṃ dīkṣitavasanājjaghanārdho vā eṣa yajñasya yatpatnī tāmetatprācīṃ yajñam prasādayiṣyanbhavatyasti vai patnyā amedhyaṃ yadavācīnaṃ nābhermedhyā vai darbhāstadyadevāsyā amedhyaṃ tadevāsyā etaddarbhairmedhyaṃ kṛtvāthainām prācīṃ yajñam prasādayati tasmānneṣṭā patnīmudānesyankauśaṃ vāsaḥ paridhāpayati kaus=aṃ vā caṇḍātakamantaraṃ dīkṣitavasanāt

Thereupon the Neshtri, being about to lead up the (Sacrificer's) wife, makes her wrap round herself, over the garment of consecration, a cloth, or skirt, made of Kusa grass; for she, the wife, is the hind part of the sacrifice; and he wishes her, thus coming forward, to propitiate the sacrifice. But impure is that part of woman which is below the navel, and pure are the plants of (Kusa) grass: thus having, by means of those plants of (Kusa) grass, made pure whatever part of her is impure, he causes her to propitiate the sacrifice, while coming forward. This is why the Neshtri, being about to lead up the wife, makes her wrap round herself, over the garment of consecration, a cloth, or skirt, made of Kusa grass.