Śatapatha Brāhmaṇa ("of a hundred paths") 1.1.4.3

1.1.4.3

saiṣā trayī vidyā yajñaḥ | tasyā etacilpameṣa varṇastadyatkṛṣṇājinam bhavati yajñasyaiva sarvatvāya tasmātkṛṣṇājinamadhi dīkṣante yajñasyaiva sarvatvāya tasmādadhyavahananamadhipeṣaṇam bhavatyaskannaṃ havirasaditi tadyadevātra taṇḍulo vā piṣṭaṃ vā skandāttadyajñe yajñaḥ pratitiṣṭhāditi tasmādadhyavahananamadhipeṣaṇam bhavati

Now this same threefold science is the sacrifice; that manifold form, that (varying) colour of this (science) is what is (represented by) this black antelope skin. For the completeness of the sacrifice (he takes the skin): hence the rite of initiation (for the Soma-sacrifice) is likewise performed on the black antelope skin;--for the completion of the sacrifice: hence it is also used for husking and bruising (the rice) on, in order that nothing of the oblation may get spilt; and that, if any grain or flour should now be spilt on it, the sacrifice would still remain securely established in the sacrifice. For this reason it is used for husking and bruising upon.