Shatapatha Brahmana 14.2.2.12

14.2.2.12

dvādaśaitāni nāmāni bhavanti dvādaśa vai māsāḥ samvatsarasya samvatsara eṣa ya eṣa tapatyeṣa u pravargyastadetamevaitatprīṇāti tasmāddvādaśa bhavanti

These are twelve names,--twelve months are in a year, and the year is he that shines yonder, and the Pravargya also is that (sun): thus it is him he thereby pleases, and therefore there are twelve (names).