Shatapatha Brahmana 11.1.2.10

11.1.2.10

ājiṃ vā ete dhāvanti ye darśapūrṇamāsābhyāṃ yajante sa vai pañcadaśa varṣāṇi yajeta teṣām pañcadaśānāṃ varṣāṇāṃ trīṇi ca śatāni ṣaṣṭiśca paurṇamāsyaścāmāvāsyāśca trīṇi ca vai śatāni ṣaṣṭiśca saṃvatsarasya rātrayastadrātrīrāpnoti

Verily, they who perform the Full and New-moon sacrifice, run a race. One ought to perform it during fifteen years;--in these fifteen years there are three hundred and sixty full moons and new moons; and there are in a year three hundred and sixty nights: it is the nights he thus gains.