Chapter 18 - Verse 7

नियतस्य तु संन्यासः कर्मणो नोपपद्यते ।
मोहात्तस्य परित्यागस्तामसः परिकीर्तितः ॥ ७ ॥

niyatasya tu saṃnyāsaḥ karmaṇo nopapadyate .
mohāttasya parityāgastāmasaḥ parikīrtitaḥ .. 7 ..


Verily, the abandonment of an obligatory duty is not proper; the abandonment thereof from ignorance is declared to be Tamasic.
Read the commentary
Not proper : since it is admitted to be a purifier in the case of an ignorant man. To hold that a duty is obligatory and then to abandon it involves a self-contradiction. Therefore, this sort of abandonment is due to ignorance and is said to be Tamasic, inasmuch a signorance is Tamas.