Bhagavad Gita Chapter 17, Verse 19

मूढग्राहेणात्मनो यत्पीडया क्रियते तपः | परस्योत्सादनार्थं वा तत्तामसमुदाहृतम् ||

mūḍha-grāheṇātmano yat pīḍayā kriyate tapaḥ parasyotsādanārthaṁ vā tat tāmasam udāhṛtam

That austerity which is performed with foolish understanding, tormenting the self, or for the purpose of destroying others—that is declared to be tamasic.

Krishna completes the description of three kinds of austerity with tamasic austerity. 'Mūḍha-grāheṇātmano yat pīḍayā kriyate tapaḥ'—that austerity (tapas) which is performed (kriyate) with foolish understanding (mūḍha-grāheṇa), tormenting the self (ātmanaḥ pīḍayā). 'Parasyotsādanārthaṁ vā'—or for the purpose of destroying others (parasya utsādanārtham). 'Tat tāmasam udāhṛtam'—that is declared (udāhṛtam) to be tamasic (tāmasam). Tamasic austerity is performed with ignorance, tormenting yourself or others, or for the purpose of harm. It's not about growth or duty—it's about ignorance and destruction. Understanding this helps you recognize and avoid tamasic austerity.