Shatapatha Brahmana 9.5.1.54

9.5.1.54

atha maitrāvaruṇyā payasyayā yajate | devatrā vā eṣa bhavati ya etatkarma karoti daivamvetanmithunaṃ yanmitrāvaruṇau sa yadetayāniṣṭvā mānuṣyāṃ caretpratyavarohaḥ sa yathā daivaḥ sanmānuṣaḥ syāttādṛktadatha yadetayā maitrāvaruṇyā payasyayā yajate daivamevaitanmithunamupaityetayeṣṭvā kāmaṃ yathāpratirūpaṃ caret

He then offers a dish of clotted curds to Mitra and Varuna. Now he who performs this (Agni-kayana) rite comes to be with the gods; and these two, Mitra and Varuna, are a divine pair. Now, were he to have intercourse with a human woman without having offered this (oblation), it would be a descent, as if one who is divine would become human; but when he offers this dish of clotted curds to Mitra and Varuna, he thereby approaches a divine mate: having offered it, he may freely have intercourse in a befitting way.