Bhagavadgita 16.83

Bhagavadgītābhāṣya (Śāṅkarabhāṣya) 16.83

tasmāc chāstraṃ pramāṇaṃ te kāryākārya-vyavasthitau |
jñātvā śāstra-vidhānoktaṃ karma kartum ihārhasi ||