Shatapatha Brahmana 5.4.5.23

5.4.5.23

tā vā etāḥ | dvādaśa vā trayodaśa vā dakṣiṇā bhavanti dvādaśa vā vai trayodaśa vā saṃvatsarasya māsāḥ saṃvatsaraḥ prajāpatiḥ prajāpatiryajñastadyajñam pratyakṣamāpnoti tam pratyakṣamāptvātmankurute

Now there are here either twelve or thirteen presents,--for either twelve or thirteen are there months 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.