Śatapatha Brāhmaṇa ("of a hundred paths") 11.2.4.7

11.2.4.7

mana eva pūrṇamāḥ pūrṇamiva hīdam mano vāgeva darśo dadṛśa iva hīyaṃ vāktadetāvadhyātmam pratyakṣaṃ darśapūrṇamāsau sa yadupavasathe vratopāyanīyamaśnāti tenaitāvadhyātmam pratyakṣaṃ darśapūrṇamāsau prīṇāti yajñena prātardaivau

The full moon is the mind, for full, as it were, is this mind; and the new moon is speech, for this speech appears, as it were. Thus these two are clearly the full and new moon, as regards the body; and inasmuch as on the day of fasting he eats the (food) suitable for eating on the vow, he thereby clearly gratifies these two in regard to the body; and on the morrow (he gratifies them) as gods by sacrifice.