Śatapatha Brāhmaṇa ("of a hundred paths") 3.7.3.1

3.7.3.1

paśuśca vai yūpaśca | na vā ṛte yūpātpaśumālabhante kadā cana tadyattathā na ha vā etasmā agre paśavaścakṣamire yadannamabhiviṣyanyathedamannam bhūtā yathā haivāyaṃ dvipātpuruṣa ucrita evaṃ haiva dvipāda ucritāśceruḥ

There are both an animal and a sacrificial stake, for never do they immolate an animal without a stake. And as to why this is so:--well, animals did not at first submit thereto that they should become food, as they are now become food; for just as man here walks two-footed and erect, so did they walk two-footed and erect.