Śatapatha Brāhmaṇa ("of a hundred paths") 4.1.4.8

4.1.4.8

tam payasā śriṇāti | tadyatpayasā śrīṇāti vṛtro vai soma āsīttaṃ yatra devā aghnaṃstam mitramabruvaṃstvamapi haṃsīti sa na cakame sarvasya vā aham mitramasmi na mitraṃ sannamitro bhaviṣyāmīti taṃ vai tvā yajñādantareṣyāma ityahamapi hanmīti hovāca tasmātpaśavo 'pākrāmanmitraṃ sannamitro 'bhūditi sa paśubhirvyārdhyata tametaddevāḥ paśubhiḥ samārdhayanyatpayasāśrīṇaṃstatho evainameṣa etatpaśubhiḥ samardhayati yatpayasā śrīṇāti

He mixes it with milk. The reason why he mixes it with milk is this. Soma, forsooth, was Vritra. Now when the gods slew him, they said to Mitra, 'Thou also slayest!' But he liked it not and said, 'Surely, I am every one's friend (mitra): being no friend, I shall become an enemy (or, other than Mitra, Amitra).'--'Then we shall exclude thee from the sacrifice!'--Then said he, 'I, too, slay!' Thereupon the cattle went from him, saying, 'Being a friend, he has become an enemy!' Thus he was deprived of the cattle. By mixing (the Soma) with milk, the gods then supplied him with cattle; and in like manner does this (priest) now supply him (the sacrificer or Mitra) with cattle by mixing (the Soma) with milk.