Chapter 17 · The Three Kinds of Faith · Krishna said
Bhagavad Gita 17.19 — Tamasic austerity
मूढग्राहेणात्मनो यत्पीडया क्रियते तपः। परस्योत्सादनार्थं वा तत्तामसमुदाहृतम् ॥
mūḍhagrāheṇātmano yatpīḍayā kriyate tapaḥ | parasyotsādanārthaṃ vā tattāmasamudāhṛtam ||
Meaning
Austerity done out of foolish stubbornness, by torturing oneself, or in order to harm others, is called tamasic.