Shatapatha Brahmana 1.6.3.34

1.6.3.34

sa vai saṃpratyevopavaset | yathā vā anyasya hataṃ saṃpiṃṣyādevaṃ tadya uttarāmupavasati so 'nyasyaiva kṛtānukaro 'nyasyopāvasāyī bhavati tasmādu saṃpratyevopavaset

Let him therefore enter on the fast at the very time (of full moon). He who keeps the fast on the following day only is as one who finally crushes one struck down by some one else; he only does what has been done before by some one else, he only follows another's lead; let him therefore enter on the fast at the very time (of full moon).