Matsyapurana 141.56

141.56

tasmātpañcadaśe some kalā vai nāsti ṣoḍaśī
tasmātsomasya viproktaḥ pañcadaśyāṃ mayā kṣayaḥ //

When the Moon separates from the Sun, one kalâ is known as the Parva period. Every day the Moon waxes by one digit when on the fifteenth day He becomes full. Hence, that day is termed Purnimâ. Then the fifteen digits of the Moon are visible. For this reason, it was stated that after the fifteenth tithi, the Moon wanes; there is no sixteenth digit of the Moon. These Devas and Pitṛis are the drinkers of Soma (Moon) and the nourishers of Soma (Moon). Ârtavas, Ṛitus and Abda, Pitṛis are the nourishers.—54-56.