Bhagavad Gita 17.19

मूढग्राहेणात्मनो यत्पीडया क्रियते तपः।परस्योत्सादनार्थं वा तत्तामसमुदाहृतम्॥

mūḍhagrāheṇātmano yatpīḍayā kriyate tapaḥ |parasyotsādanārthaṃ vā tattāmasamudāhṛtam ||

That austerity which is practised out of a foolish notion, with self-torture, or for the purpose of destroying another, is declared to be of the Tamasic nature.

Austerity done under delusion, and accompanied with sorcery or the infliction of pain upon oneself or another, may be assumed to spring from ignorance.

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