Śatapatha Brāhmaṇa ("of a hundred paths") 3.3.4.28

3.3.4.28

nābhidaghnā bhavati | atra vā annam pratitiṣṭhanyannaṃ somastasmānnābhidaghnā bhavatyatro eva retasa āśayo retaḥ somastasmādatradaghnā bhavati

It reaches up to his navel, for it is there that the food settles, and Soma is food: therefore it reaches up to his navel. Moreover, there is the seat of the seed, and Soma is seed: therefore it reaches up to his navel.