Śatapatha Brāhmaṇa ("of a hundred paths") 3.7.1.19

3.7.1.19

atha parivyayati | anagnatāyai nveva parivyayati tasmādatreva parivyayatyatreva hīdaṃ vāso bhavatyannādyamevāsminnetaddadhātyatreva hīdamannam pratitiṣṭhati tasmādatreva parivyayati

He then girds (the stake with a rope of kusa-grass). Now it is to cover its nakedness that he girds it; wherefore he girds it in this place (viz. on a level with the sacrificer's navel), for it is thus that this (nether) garment is (slung round). He thereby puts food into him, for it is there that the food settles; therefore he girds it at that place.