Śatapatha Brāhmaṇa ("of a hundred paths") 2.5.3.4

2.5.3.4

atha marudbhyo gṛhamedhibhyaḥ | śākhayā vatsānapākatya pavitravati saṃdohya taṃ caruṃ śrapayati caruru hyeva sa yatra kva ca taṇḍulānāvapanti tanmedho devā dadhire prātarvṛtraṃ haniṣyantastatho evaiṣa etatpāpmānaṃ dviṣantam bhrātṛvyaṃ haniṣyanmegho dhatte tadyatkṣīraudano bhavati megho vai payo meghastaṇḍulāstamubhayam meghamātmandhatte tasmātkṣīraudano bhavati

Thereupon, (in the evening, he offers a potful of boiled grain) to the Maruts, the Householders (Grihamedhinah). That pap he cooks after driving away the calves (from the cows) with the (palâsa-) branch, and having (all the cows) milked into the pot containing the strainers. Now, whenever (in preparing the pap) they use (whole) rice-grains, then that is a karu: this nourishment the gods took when they were about to slay Vritra on the morrow; and so does he (the Sacrificer) now take that nourishment, being about to slay his wicked, spiteful enemy. The reason, then, why it is milk-pap, is that milk is nourishment, and rice-grains are nourishment, and that he thus puts into him (âtman) that twofold nourishment. For this reason it is a rice-pap (prepared) with milk.