Śatapatha Brāhmaṇa ("of a hundred paths") 14.1.3.28

14.1.3.28

trayodaśa sampādayati trayodaśa vai māsāḥ samvatsarasya samvatsara eṣa ya eṣa tapatyeṣa u pravargyastadetamevaitatprīṇāti tasmāttrayodaśa sampādayati

He makes them to amount to thirteen, for there are thirteen months in the year, and the year is he who shines yonder, and the Pravargya also is that (sun): it is him he thus gratifies, and hence he makes them to amount to thirteen.