Śatapatha Brāhmaṇa ("of a hundred paths") 3.2.2.14

3.2.2.14

taddhaike | prathame vrata ubhau vrīhiyavāvāvapantyubhābhyāṃ rasābhyāṃ yadevātra yajñasya nirdhītaṃ yadvidugdhaṃ tatpunarāpyāyayāma iti vadanto yadyu vratadughā na duhīta yasyaivātaḥ kāmayeta tasya vrataṃ kuryādetadu hyevāsyaitā ubhau vrīhiyavāvanvārabdhau bhavata iti tadu tathā na kuryānna ha sa yajñamāpyāyayati na saṃdadhāti ya ubhau vrīhiyavāvāvapati tasmādanyataramevāvapeddhavirvā asyaitā ubh au vrīhiyavau bhavataḥ sa yadevāsyaitau havirbhavatastadevāsyaitāvanvārabdhau bhavato yadyu vratadughā na duhīta yasyaivātaḥ kāmayeta tasya vrataṃ kuryāt

Now, some add both rice and barley to the first (day's) fast-milk, arguing,--"By means of these two substances (rasa) we restore what part of the sacrifice was sucked out and drained; and, should the vrata-cow yield no milk, he may prepare his fast-food of whichever of these (cereals) he pleases; and thus both the rice and the barley are 'taken hold of' by him." But let him not do this; for he who adds both rice and barley (to the milk) neither replenishes the sacrifice nor heals it. Let him therefore add only the one or the other (cereal). Both the rice and barley doubtless form his (havis) material for offering (at the New and Full-moon Sacrifice), and when they do so become his material for offering, then they are also 'taken hold of ' by him. Should the vrata, cow yield no milk, let him prepare the fast-food of whichever of them he pleases.