Shatapatha Brahmana 1.6.3.37

1.6.3.37

sa saṃhitaiḥ parvābhiḥ | idamannādyamabhyuttasthau yadidam prajāpaterannādyaṃ sa yo haivaṃ vidvāntsaṃpratyupavasati saṃprati haiva prajāpateḥ parva bhiṣajyatyavati hainaṃ prajāpatiḥ sa evamevānnādo bhavati ya evaṃ vidvāntsaṃpratyupavasati tasmādu saṃpratyevopavaset

With his joints thus repaired he betook himself to this food,--to the food which is here (offered) to Pragâpati; and he who, knowing this, enters upon the fast at the very time (of full moon), heals Pragâpati's joint at the proper time, and Pragâpati favours him. Thus he who, knowing this, enters upon the fast at the very time (of full moon) becomes a consumer of food: let him therefore enter on the fast at the very time (of full moon).