Shatapatha Brahmana 10.4.2.17

10.4.2.17

caturviṃśatimātmano'kuruta triṃśadiṣṭakāntso'trātiṣṭhata pañcadaśe vyūhe tadyatpañcadaśe vyūhe'tiṣṭhata tasmātpañcadaśāpūryamāṇasya rūpāṇi pañcadaśāpakṣīyamāṇasya

He made himself twenty-four bodies of thirty bricks each. There he stopped, at the fifteenth; and because he stopped at the fifteenth arrangement there are fifteen forms of the waxing, and fifteen of the waning (moon).