Śatapatha Brāhmaṇa ("of a hundred paths") 13.3.8.5

13.3.8.5

atha yadyudake mriyeta vāruṇaṃ yavamayaṃ carumanunirvapedvaruṇo vā etaṃ gṛhṇāti yo'psu mriyate sā yaivainaṃ devatā gṛhṇāti tāmevaitatprīṇāti sāsmai prītānyamālambhāyānumanyate tayānumatamālabhate sa yadyavamayo bhavati varuṇyā hi yavāḥ

And if it were to die in water, let him in addition prepare a barley pap to Varuna, for Varuna seizes him who dies in water: he thereby thus gratifies that very deity who seizes it, and, thus gratified, he approves his slaughtering another (horse), and he slaughters it as one approved by that (deity). And as to why it is (prepared) of barley, it is because barley belongs to Varuna.