Shatapatha Brahmana 6.7.1.10

6.7.1.10

yadvevoparinābhi | etadvai paśormedhyataraṃ yaduparinābhi purīṣasaṃhitataraṃ yadavāṅnābhestadyadeva paśormedhyataraṃ tenainametadbibharti

And, again, why over the navel;--sacrificially purer is that part of the animal (victim) which is above the navel, and more in contact with ordure is that which is below the navel: he thus carries it (the plate) by means of that part of the animal which is sacrificially purer.