Śatapatha Brāhmaṇa ("of a hundred paths") 4.5.2.2

4.5.2.2

na vai tadavakalpate | yadekām manyamānā ekayevaitayā careyuryaddve manyamānā dvābhyāmiva careyu sthālīṃ caivoṣṇīṣaṃ copakalpayitavai brūyāt

For surely it is not right that, thinking it to be one (cow), they should perform, as it were, with that one; or that, thinking them to be two, they should perform, as it were, with two. Let him bid (the slaughterer) get ready the pot (sthâlî) and the cloth (ushnîsha).