Shatapatha Brahmana 6.2.3.1

6.2.3.1

etadvai devā abruvan | cetayadhvamiti citimicateti vāva tadabruvaṃsteṣāṃ cetayamānānām prajāpatirimām prathamāṃ svayamātṛṇāṃ citimapaśyattasmāttām prajāpatinopadadhāti

Now, the gods said, 'Meditate ye!'--whereby, no doubt, they meant to say, 'Seek ye a layer (for the fire-altar)! 'Whilst they were meditating, Pragâpati saw this earth, as a first naturally-perforated layer: hence it is by means of Pragâpati that he lays on that (brick).