Shatapatha Brahmana 5.4.5.20

5.4.5.20

tasya dvādaśa prathamagarbhāḥ paṣṭhauhyo dakṣiṇā | dvādaśa vai māsāḥ saṃvatsarasya saṃvatsaraḥ prajāpatiḥ prajāpatiryajñastadyajñam pratyakṣamāpnoti tam pratyakṣamāptvātmankurute

Twelve heifers with first calf are the sacrificial fee for this (sacrifice); for twelve months there are in the year, and the year is Pragâpati, and Pragâpati is the sacrifice: thus he visibly obtains the sacrifice, and having visibly obtained it, he makes it his own.