CHĀNDOGYA UPANISHAD · 3.17.3
अथ यद् धसति यज् जक्षति यन् मैथुनं चरति स्तुतशस्त्रैर् एव तद् एति ॥
atha yad dhasati yaj jakṣati yan maithunaṃ carati stutaśastrair eva tad eti ॥
When a man laughs, eats, and delights himself, he does it with the Stuta-sastras (hymns sung and recited at the sacrifices).
F. Max Müller · The Upanishads, Sacred Books of the East 1 (1879)