Kauṣītaki (Śāṅkhāyana) Brāhmaṇa 3.1.3

3.1.3

tasmād upavasati / (DpM)

iii. 1. In that he fasts at the new and full moon it is because the gods do not eat the oblation of him who does not perform the fast. Therefore he fasts (desiring) 'May the gods eat my oblation'. 'On the first full moon (day) should he fast' is the view of Paiṅgya; 'on the second' that of Kauṣītaki; 'that on which (the moon) appears (full) about the setting of the sun' is the rule; he should fast on the second full moon day.