Verse
मूढग्राहेणात्मनो यत् पीडया क्रियते तपः ।परस्योत्सादनार्थं वा तत्तामसमुदाहृतम् ॥
That askesis which is pursued under a clouded and deluded idea, performed with effort and suffering imposed on oneself or else with a concentration of the energy in a will to do hurt to others, that is said to be tamasic.
Gītā 17·19