Śatapatha Brāhmaṇa ("of a hundred paths") 1.1.1.7

1.1.1.7

athāto 'śanānaśanasyaiva | taduhāṣāḍhaḥ sāvayaso 'naśanameva vratam mene mano ha vai devā manuṣyasyājānanti ta enametadvratamupayantaṃ viduḥ prātarno yakṣyata iti te 'sya viśve devā gṛhānāgacanti te 'sya gṛheṣūpavasanti sa upavasathaḥ

Now then of the eating (or) fasting. And on this point Âshâdha Sâvayasa, on the one hand, was of opinion that the vow consisted in fasting. For assuredly, (he argued,) the gods see through the mind of man; they know that, when he enters on this vow, he means to sacrifice to them the next morning. Therefore all the gods betake themselves to his house, and abide by (him or the fires, upa-vas) in his house; whence this (day) is called upa-vasatha.