Śatapatha Brāhmaṇa ("of a hundred paths") 5.5.5.19

5.5.5.19

tā vā etāḥ | dvādaśa vā trayodaśa vā dakṣiṇā bhavanti dvādaśa vā vai trayodaśa vā saṃvatsarasya māsāḥ saṃvatsarasammitaiṣeṣṭistasmāddvādaśa vā trayodaśa vā dakṣiṇā bhavanti

Now there are here either twelve, or thirteen gifts, and there are either twelve or thirteen months in the year;--the offering thus is of equal measure with the year: that is why there are either twelve or thirteen sacrificial gifts.