Shatapatha Brahmana 1.3.1.13

1.3.1.13

yoktreṇa saṃnahyati | yoktreṇa hi yogyaṃ yuñjantyasti vai patnyā amedhyaṃ yadavācīnaṃ nābherathaitadājyamavekṣiṣyamāṇā bhavati tadevāsyā etadyoktreṇāntardadhātyatha medhyenaivottarārdhanājyamavekṣate tasmātpatnīṃ saṃnahyati

He girds her with a cord (yoktra): for with a cord (yoktra) they yoke the draught-animal (yogya). Impure indeed is that part of woman which is below the navel; and therewith she will be facing the sacrificial butter: that part of her he thereby conceals with the cord, and only with the pure upper part of her body she then faces the sacrificial butter. This is the reason why he girds the wife.