Shatapatha Brahmana 4.5.1.7

4.5.1.7

tadāhuḥ | kve jāno 'bhūditi tadyadevāsyātra mitraḥ sviṣṭaṃ gṛhṇāti tadevāsmā etayā prītaḥ pratyavasṛjati yadu cāsya varuṇo duriṣṭaṃ gṛhṇāti taccaivāsmā etayā prītaḥ sviṣṭaṃ karoti tadu cāsmai pratyavasṛjati so 'syaiṣa sva eva yajño bhavati svaṃ sukṛtam

Then they say, 'What has become of the sacrificer?'--whatever well-offered part of his (sacrifice) Mitra here takes, that he now again surrenders to him, being pleased with this (cow); and whatever ill-offered part of his Varuna takes, that indeed he makes well-offered for him, being pleased with this (cow), and surrenders it again to him. This forsooth is his own sacrifice, his own merit.