Śatapatha Brāhmaṇa ("of a hundred paths") 5.5.4.1

5.5.4.1

śyeta āśvino bhavati | śyetāviva hyaśvināvavirmalhā sārasvatī bhavatyṛṣabhamindrāya sṛtrāmṇa ālabhate durvedā evaṃsamṛddhāḥ paśavo yadyevaṃsamṛddhānna vindedapyajānevālabheraṃste hi suśrapatarā bhavanti sa yadyajānālabheraṃlohita āśvino bhavati tadyadetayā yajate

There is a reddish-white (he-goat as the victim) for the Asvins, for the Asvins are reddish-white. There is an ewe with teats in the dewlap for Sarasvatî; and a bull he seizes for Indra Sutrâman (the good protector). Difficult to obtain are beasts with such perfections; if he cannot obtain any with such perfections, they may slaughter only goats, for they are easier to cook. And if they seize only goats, that for the Asvins is a red one. Then as to why he performs this sacrifice.