Shatapatha Brahmana 5.4.5.21

5.4.5.21

tāsāṃ dvādaśa garbhāḥ | tāścaturviṃśatiścaturviṃśatirvai saṃvatsarasyārdhamāsāḥ saṃvatsaraḥ prajāpatiḥ prajāpatiryajñastadyajñam pratyakṣamāpnoti tam pratyakṣamāptvātmankurute

These (heifers) have twelve embryo calves,--that makes twenty-four; for twenty-four half-moons 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.