Śatapatha Brāhmaṇa ("of a hundred paths") 10.5.2.11

10.5.2.11

athādhiyajñam yade tanmaṇḍalaṃ tapatyayaṃ sa rukmo'tha yadetadarcirdīpyata idaṃ tatpuṣkaraparṇamāpo hyetā āpaḥ puṣkaraparṇamatha ya eṣa etasminmaṇḍale puruṣo'yameva sa yo'yaṃ hiraṇmayaḥ puruṣastadetadevaitattrayaṃ saṃskṛtyehopadhatte tadyajñasyaivānu saṃsthāmūrdhvamutkrāmati tadetamapyeti ya eṣa tapati tasmādagniṃ nādriyeta parihantumamutra hyeṣa tadā bhavatītyu evādhiyajñam

Those two (persons in the eyes) descend to the cavity of the heart, and enter into union with each other; and when they reach the end of their union, then the man sleeps,--even as here on reaching the end of a human union he becomes, as it were, insensible, so does he then become, as it were, insensible; for this is a divine union, and that is the highest bliss.