Shatapatha Brahmana 11.1.7.1

11.1.7.1

tadvā ado vratopāyana udyate yadi nāśnāti pitṛdevatyo bhavati yadyu aśnāti devānatyaśnātīti tadāraṇyamaśnīyāditi tatra sthāpayanti

Now there, on the occasion of the entering on the fast, it is said, 'If he does not eat, he becomes consecrated to the Fathers; and if he does eat he eats whilst passing over the gods;' and, in this respect, they lay down the rule, 'Let him therefore eat what grows in the forest.'