Shatapatha Brahmana 9.5.1.56

9.5.1.56

sa yaḥ sa prajāpatirvyasraṃsata | ayameva sa yo 'yamagniścīyate 'tha yadasmāttadretaḥ parāpatadeṣā sā payasyā maitrāvaruṇī bhavati prāṇodānau vai mitrāvaruṇau prāṇodānā u vai retaḥ siktaṃ vikurutaḥ payasyā bhavati payo hi reto yajño bhavati yajño hyeva yajñasya reta upāṃśu bhavatyupāṃśu hi retaḥ sicyate 'ntato bhavatyantato hi reto dhīyate

Now that Pragâpati who became released is this very fire-altar which is here being built; and the seed which fell from him is this dish of clotted curds of Mitra and Varuna; for Mitra and Varuna are the in-breathing and up-breathing, and the in-breathing and up-breathing fashion the infused seed. A dish of clotted curds it is, because seed is milk; and sacrifice it is, because sacrifice is the seed of sacrifice. In a low voice it is (offered), for silently seed is shed. At the end (of the sacrifice) it is (offered), for from the end seed is introduced.