Śatapatha Brāhmaṇa ("of a hundred paths") 4.2.1.11

4.2.1.11

taṃ saktubhiḥ śrīṇāti | tadyatsaktubhiḥ śrīṇāti varuṇo ha vai somasya rājño 'bhīvākṣi pratipipeṣa tadaśvayattato 'śvaḥ samabhavattadyacvayathātsamabhavattasmādaśvo nāma tasyāśru prāskandattato yavaḥ samabhavattasmādāhurvaruṇyo yava iti tadyadevāsyātra cakṣuṣo 'mīyata tenaivainametatsamardhayati kṛtsnaṃ karoti tasmātsaktubhiḥ śrīṇāti

He mixes it with (barley) meal: the reason why he mixes it with meal is this. Varuna once struck king Soma right in the eye, and it swelled (asvayat): therefrom a horse (asva) sprung; and because it sprung from a swelling, therefore it is called asva. A tear of his fell down: therefrom the barley sprung; whence they say that the barley belongs to Varuna. Thus whatever part of his eye was injured on that occasion in (that part he now restores him and makes him whole by means of this (barley): therefore he mixes (the libation) with meal.