Shatapatha Brahmana 10.2.5.5

10.2.5.5

atha yadi caturviṃśatiḥ caturviṃśatirvā ardhamāsā ardhamāsā upasada ādityaḥ pravargyo'muṃ tadādityamardhamāseṣu pratiṣṭhāpayati tasmādeṣo'rdhamāseṣu pratiṣṭhitaḥ

And 'if there are twenty-four (Upasad-days), there being twenty-four half moons--the Upasads are the half-moons, and the Pravargya is the sun: he thus establishes yonder sun on the half-moons, whence he is established on the half-moons.