Śatapatha Brāhmaṇa ("of a hundred paths") 5.2.1.24

5.2.1.24

athājarṣabhasyājinamāstṛṇāti | prajāpatirvā eṣa yadajarṣabha etā vai prajāpateḥ pratyakṣatamāṃ yadajāstasmādetāstriḥ saṃvatsarasya vijāyamānā dvau trīniti janayanti tatprajāpatimevaitatkaroti tasmādajarṣabasyājinamāstṛṇāti

He then spreads the goat-skin thereon; for truly the he-goat is no other than Pragâpati, for they, the goats, are most clearly of Pragâpati (the lord of generation or creatures);--whence, bringing forth thrice in a year, they produce two or three: thus he thereby makes him (the Sacrificer) to be Pragâpati himself,--this is why he spreads the goat-skin thereon.