Jaiminīya (Talavakāra) Brāhmaṇa 2-3.108.p6

2-3.108.p6

atha yatpañcadaśa stotrāṇi bhavanti pañcadaśa vārdhamāsasya rātrayastenārdhamāsaśaḥ

Then, because there are fifteen chants—fifteen indeed are the nights of a half-month—thereby [it is sought] half-month by half-month.