Srimad-Bhagavatam: Canto 3 - Chapter 1 - Verse 10
Sanskrit:
यामाश्चत्वारश्चत्वारो मर्त्यानामहनी उभे ।पक्ष: पञ्चदशाहानि शुक्ल: कृष्णश्च मानद ॥ १० ॥
ITRANS:
yāmāś catvāraś catvāromartyānām ahanī ubhepakṣaḥ pañca-daśāhāniśuklaḥ kṛṣṇaś ca mānada
Translation:
It is calculated that there are four praharas, which are also called yāmas, in the day and four in the night of the human being. Similarly, fifteen days and nights are a fortnight, and there are two fortnights, white and black, in a month.