Śatapatha Brāhmaṇa ("of a hundred paths") 4.1.4.9

4.1.4.9

tadāhuḥ | śaśvaddha naiva cakame hantumiti tadyadevātra payastanmitrasya soma eva varuṇasya tasmātpayasā śrīṇāti

As to this they say, 'Surely he liked it not to slay!' Thus, what milk there is in this (mixture) that belongs to Mitra, but the Soma belongs to Varuna: therefore one mixes it with milk.