Śatapatha Brāhmaṇa ("of a hundred paths") 3.9.2.7

3.9.2.7

etaddha vai viśve devāḥ | yajamānasya gṛhānāgacanti sa yaḥ purādityasyāstamayādvasatīvarīrgṛhṇāti yathā śreyasyāgamiṣyatyāvasathenopakLptenopāsītaivaṃ tatta etaddhaviḥ praviśanti ta etāsu vasatīvarīṣūpavasanti sa upavasathaḥ

And again, all the gods, forsooth, now come to the sacrificer's house; and if one takes the Vasatîvarî water before the setting of the sun, it is as when one's better comes to visit one, he would honour him by trimming his house. They draw nigh to that sacrificial food, and abide (upa-vas) in that Vasatîvarî water,--that is the Upavasatha (preparation-day).