YV 1.2n

It is black, for it has Varuna as its deity; (verily it serves) for prosperity. He who desires food should offer a white (beast) to Mitra and a black to Varuna at the union of the waters and the plants; the plants are connected with Mitra, and the waters with Varuna; on the sap of the water and of the plants do we live; verily lie has recourse to Mitra and Varuna with their own share; verily they give him food; he becomes an eater of food [2]. He should offer at the union of the waters and of the plants, to attain both. The sacrificial post is bifurcate, for there are two deities; (verily it serves) for prosperity. He who is long ill should offer a white (beast) to Mitra, and a black to Varuna; in that one is offered to Mitra, by means of Mitra he appeases Varuna for him; in that one is offered to Varuna, straightway he sets him free from Varuna's noose; even if his life be gone, he yet lives. The gods could not find prosperity [3]; they saw it in the pair; they could not agree about it; the Açvins said, 'Ours is it; do not claim it.' It became the Açvins' only. He who desires prosperity should offer to the Açvins a twin cow; verily he has recourse to the Açvins with their own share; verily they bestow prosperity upon him; he prospers in offspring and cattle. He who being a bad Brahman desires to drink Soma should offer to the Açvins a dusky (beast) with spots on the forehead; the Açvins were among the gods those who did not drink Soma; they later acquired the drinking of Sonia; the Açvins are the gods of the bad Brahman who desires to drink Soma; verily he has recourse to the Açvins with their own share; verily they give to him the drinking of Soma; the drinking of Soma comes to him. In that it is dusky, verily he drives away the darkness from him. In that it has spots on the forehead [1], verily at the beginning he bestows brilliance on him.