Śatapatha Brāhmaṇa ("of a hundred paths") 7.5.2.43

7.5.2.43

tadāhuḥ | yadimā āpa etāni māṃsānyatha kva tvakvva lometyannaṃ vāva paśostvagannaṃ loma tadyaccandasyā upadadhāti saiva paśostvaktallomātho yānyamūnyukhāyāmajalomāni tāni lomāni bāhyokhā bhavatyantarāṇi paśuśīrṣāṇi bāhyāni hi lomānyantara ātmā yadītareṇa yadītareṇeti ha smāha śāṇḍilyaḥ sarvāneva vayaṃ kṛtsnānpaśūntsaṃskurma iti

As to this they say, 'If there are that water and that flesh, where then is the skin, and where the hair?' Well, the skin of cattle is food, and the hair of cattle is food; and when he lays down the Khandasyâh, that is the skin of the victims, that is their hair. Or, again, those goats’ hair which are in the fire-pan, they are hair. The fire-pan is outside, and the victims’ heads are inside, for outside is the hair, and inside is the body. 'Whether in the one way, or whether in the other,' so Sândilya was wont to say, 'in any case we make up the victims wholly and completely.'