Bhagavad Gita, Chapter 3, Verse 15

Know Karma to have risen from the Veda, and the Veda from the Imperishable. Therefore the all-pervading Veda is ever centred in Yajna.

Even the instructions themselves aren't separate from what they're pointing toward.

This verse closes the loop on where duty actually comes from. It's not an arbitrary set of rules, it traces back to the same source as everything else, which is exactly why doing it with real intention matters.