Śatapatha Brāhmaṇa ("of a hundred paths") 2.1.4.2

2.1.4.2

tannvevānavakLptaṃ yo manuṣyeṣvanaśnatsu pūrvo 'śnīyādatha kimu yo deveṣvanaśnatsu pūrvo 'śnīyāttasmādu divaivāśnīyāttadvapi kāmameva naktamaśnīyānno hyanāhitāgnervratacaryāsti mānuṣo hyevaiṣa tāvadbhavati yāvadanāhitāgnistasmādvapi kāmameva naktamaśnīyāt

Now, as it would be unbecoming for him to take food before men (who are staying with him as his guests) have eaten; how much more would it be so, if he were to take food before the gods have eaten: let him therefore take his food in the day-time. However, he may also, if he choose, take food at night, since the observance of the vow is not necessary for him who has not performed Agnyâdheya. For so long as he has not set up a (sacrificial) fire of his own, he is merely a man, and may therefore, if he choose, take food at night.