Mahabharata 12.80.2

भीष्म उवाच · bhīṣma uvāca

प्रतिकर्म पुराचार ऋत्विजां स्म विधीयतेआदौ छन्दांसि विज्ञाय द्विजानां श्रुतमेव च

pratikarma purācāra ṛtvijāṃ sma vidhīyateādau chandāṃsi vijñāya dvijānāṃ śrutameva ca

Our plain-language meaning of this verse is waiting for scholar review.