Kauṣītaki (Śāṅkhāyana) Brāhmaṇa 18.8.17

18.8.17

atha yad apsu varuṇam yajati /

xviii. 11. The Anūbandhyā (is explained). The offering of the cow is the fourth pressing; therefore is it immovable, for it is the fourth of the pressings. It is offered to Mitra and Varuṇa, for (the victim) for Agni and Soma has been offered before; therefore is it for Mitra and Varuṇa, to secure the equipoise of the sacrifice. Moreover, in that he sacrifices to Varuṇa in the waters, therein is Mitra neglected; therefore is it for Mitra and Varuṇa, to secure Mitra also.