Śatapatha Brāhmaṇa ("of a hundred paths") 4.5.1.6

4.5.1.6

tadyanmaitrāvaruṇī vaśā bhavati | yadvā ījānasya sviṣṭam bhavati mitro 'sya tadgṛhṇāti yadvasya duriṣṭam bhavati varuṇo 'sya tadgṛhṇāti

The reason why there is a barren cow for Mitra and Varuna is this. Whatever part of his (sacrifice) who has offered is well-offered that part of his Mitra takes, and whatever is ill-offered that Varuna takes.