Jaiminīya (Talavakāra) Brāhmaṇa 2-3.108.p7

2-3.108.p7

atha yattriṃśatstutaśastrāṇi bhavanti triṃśanmāsasya rātrayastena māsaśaḥ

Then, because there are thirty chant-and-recitations—thirty are the nights of a month—thereby [it is sought] month by month.