Śatapatha Brāhmaṇa ("of a hundred paths") 1.6.3.33

1.6.3.33

sa vā uttarāmevopavaset | samiva vā eṣa kramate yaḥ saṃpratyupavasatyanaddhā vai saṃkrāntayoryadītaro vetaramabhibhavatītaro vetaramatha ya uttarāmupavasati yathā parāñcamāvṛttaṃ saṃpiṃṣyādapratyālabhamānaṃ so 'nyatoghātyeva syādevaṃ tadya uttarāmupavasati

One may also fast only on the following day. Now he who (enters on the) fast at the very time (of full moon), gets, as it were, into collision (with some one); and when two come into collision with one another, it is indeed doubtful which of the two will get the better of the other. He, on the other hand, who prefers to fast on the second day (only), is as one who crushes from behind a retreating (enemy) before he is able to resist the attack: striking in one direction, in fact, is he who thus keeps the fast on the second day only.