Mudgalapurana 3.15.41

3.15.41

candracihnena viprarṣe jñātavyaṃ vibudhaiḥ sadā |
anyacca pitṛṇāṃ kālaścandramāsena jāyate ||

By the sign of the moon it is always to be understood by the wise, O brahmin sage; / moreover, the time of the ancestors comes to be by the lunar month.