Bhagavad Gita · Chapter 16: The Divine and the Demonic Natures
Bhagavad Gita 16.24
तस्माच्छास्त्रं प्रमाणं ते कार्याकार्यव्यवस्थितौ ।
ज्ञात्वा शास्त्रविधानोक्तं कर्म कर्तुमिहार्हसि ॥
tasmācchāstraṃ pramāṇaṃ te kāryākāryavyavasthitau | jñātvā śāstravidhānoktaṃ karma kartumihārhasi ||
So let scripture be your authority in deciding what should be done and what should not. Knowing what scripture prescribes, you should do your work in this world.
Modern context
Let scripture be your guide in deciding what to do. This verse is sometimes read as a call to rigid rule-following. But in the context of the whole Gita, it is more a call to humility: do not trust your desires alone; anchor your choices in something tested and wise. Krishna has spent sixteen chapters explaining the reasons behind dharma. He is asking Arjuna to act from understanding, not impulse.