CHĀNDOGYA UPANISHAD · 3.17.2
अथ यद् अश्नाति यत् पिबति यद् रमते तद् उपसदैर् एति ॥
atha yad aśnāti yat pibati yad ramate tad upasadair eti ॥
When a man eats, drinks, and enjoys pleasures, he does it with the Upasadas (the sacrificial days on which the sacrificer is allowed to partake of food).
F. Max Müller · The Upanishads, Sacred Books of the East 1 (1879)