Śatapatha Brāhmaṇa ("of a hundred paths") 1.1.1.8

1.1.1.8

tannvevānavakLptam | yo manuṣyeṣvanaśnatsu pūrvo 'śnīyādatha kimu yo deveṣvanaśnatsu pūrvo 'śnīyāttasmādu naivāśnīyāt

Now, as it would even 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 (who are staying with him) have eaten: let him therefore take no food at all.