Śatapatha Brāhmaṇa ("of a hundred paths") 10.2.2.1

10.2.2.1

yānvai tāntsapta puruṣān ekam puruṣamakurvantsa prajāpatirabhavatsa prajā asṛjata sa prajāḥ sṛṣṭvordhva udakrāmatsa etaṃ lokamagacadyatraiṣa etattapati no ha tarhyanya etasmādatra yajñiya āsa taṃ devā yajñenaiva yaṣṭumadhriyanta

Now the one person which they made out of those seven persons became this Pragâpati. He produced living beings (or offspring), and having produced living beings he went upwards,--he went to that world where that (sun) now shines. And, indeed, there was then no other (victim) meet for sacrifice but that one (Pragâpati), and the gods set about offering him up in sacrifice.