Shatapatha Brahmana 11.1.6.34

11.1.6.34

asthyeva yājyānuvākyāḥ māṃsaṃ havistanmitaṃ cando yadyājyānuvākyāstasmādu samāvantyevāsthīni medyataśca kṛśyataśca bhavantyatha yadbhūya iva ca havirgṛhṇāti kanīya iva ca tasmādu māṃsānyeva medyato medyanti māṃsāni kṛśyataḥ kṛśyanti tenaitena yajñena yāṃ kāmayate devatāṃ tāṃ yajati yasyai havirbhavati

The invitatory and offering-formulas are the bone, and the offering-material is the flesh. The invitatory and offering-formulas are (in) measured metre, whence the bones of a fat and a lean person are alike: but inasmuch as he takes now more, now less, offering-material, therefore the flesh of a fat person is fat, and the flesh of a lean person is lean. This sacrifice he performs to any deity he pleases and for whom there is a sacrificial dish.