Mahabharata 12.236.15

चतुर्थश्चौपनिषदो धर्मः साधारणः स्मृतःवानप्रस्थो गृहस्थश्च ततोऽन्यः संप्रवर्तते

caturthaścaupaniṣado dharmaḥ sādhāraṇaḥ smṛtaḥvānaprastho gṛhasthaśca tato'nyaḥ saṃpravartate

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