Śatapatha Brāhmaṇa ("of a hundred paths") 3.7.3.2

3.7.3.2

tato devā etaṃ vajraṃ dadṛśuḥ | yadyūpaṃ tamuciśriyustasmādbhīṣā prāvlīyanta tataścatuṣpādā abhavaṃstato 'nnamabhavanyathedamannam bhūtā etasmai hi vā ete 'tiṣṭhanta tasmādyūpa eva paśumālabhante narte yūpātkadā cana

Then the gods perceived that thunderbolt, to wit, the sacrificial stake; they raised it, and from fear thereof they (the animals) shrunk together and thus became four-footed, and thus became food, as they are now become food, for they submitted thereto: wherefore they immolate the animal only at a stake and never without a stake.