Śatapatha Brāhmaṇa ("of a hundred paths") 6.2.1.39

6.2.1.39

mṛnmayānyu haike kurvanti | utsannā vā ete paśavo yadvai kiṃcotsannamiyaṃ tasya sarvasya pratiṣṭhā tadyatraite paśavo gatāstata enānadhi sambharāma iti na tathā kuryādyo vā eteṣāmāvṛtaṃ ca brāhmaṇaṃ ca na vidyāttasyaita utsannāḥ syuḥ sa etāneva pañca paśūnālabheta yāvadasya vaśaḥ syāttānhaitānprajāpatiḥ prathama ālebhe śyāparṇaḥ sāyakāyano 'ntamo 'tha ha smaitānevāntareṇālabhante 'thaitarhīmau dvāvevālabhyete prājāpatyaśca vāyavyaśca tayorato brāhmaṇamudyate

Some, again, make earthen ones, thinking, 'Passed away, forsooth, are these animals, and this earth is the shelter of all that has passed away: thus whither those animals have gone, from thence we collect them.' Let him not do so, for whoso knows not both the practice and theory of these (victims), for him let them be passed away. Let him slaughter those very five victims, as far as he may be able to do so; for it was these Pragâpati was the first to slaughter, and Syâparna Sâyakâyana the last; and in the interval also people used to slaughter them. But nowadays only these two are slaughtered, the one for Pragâpati, and the one for Vâyu. The theory of these two is now (to be) told.