Śatapatha Brāhmaṇa ("of a hundred paths") 10.2.5.6

10.2.5.6

atha yadi dvādaśa dvādaśa vai māsā māsā upasada ādityaḥ pravargyo'muṃ tadādityam māseṣu pratiṣṭhāpayati tasmādeṣa māseṣu pratiṣṭhitaḥ

And if there are twelve (Upasad-days) --there being twelve months--the Upasads are the months, and the Pravargya is the sun: he thus establishes yonder sun on the months, whence he is established on the months.