Śatapatha Brāhmaṇa ("of a hundred paths") 2.4.4.10

2.4.4.10

atha prātaḥ | āgneyaḥ puroḍāśo bhavati maitrāvaruṇī payasyā nedyajñādayānīti nvevāgneyaḥ puroḍāśo 'thaitāveva mitrāvaruṇau dve devate dve vai mithunam mithunamevaitatprajananaṃ kriyata etadu hāsya tadrūpaṃ yena bahurbhavati yena prajāyate

Then on the morrow there are Agni's cake and Mitra and Varuna's curds. Now Agni's cake (is offered), for the sole purpose that it may not forsake the sacrifice. Then those two, Mitra and Varuna, are two deities, and two means a pair: hence a productive pair is thereby obtained; and thus is (produced) that form (of the sacrifice) whereby he becomes many, whereby he is reproduced.